Try to eval "skeleton" of file with error

The resulting REPL has a namespace with bindings from module
languages, requires, and definitions. That way things like completion
and racket-describe are more likely to work while you are editing the
file to fix the error.

Closes #272.

Reference.md

@@ -23,33 +23,43 @@
 
 Save and evaluate the buffer in REPL, much like DrRacket's Run.
 
-If point is within a submodule form, the REPL "enters" that
-submodule (uses its language info and namespace).
+With a C-u prefix, uses errortrace for improved stack traces.
+Otherwise follows the [`racket-error-context`](#racket-error-context) setting.
+
+If point is within a Racket `module` form, the REPL "enters"
+that submodule (uses its language info and namespace).
 
 When you run again, the file is evaluated from scratch -- the
 custodian releases resources like threads and the evaluation
 environment is reset to the contents of the file. In other words,
 like DrRacket, this provides the predictability of a "static"
-baseline, plus the ability to explore interatively using the
+baseline, plus the ability to explore interactively using the
 REPL.
 
 See also [`racket-run-and-switch-to-repl`](#racket-run-and-switch-to-repl), which is even more like
 DrRacket's Run because it selects the REPL window (gives it the
 focus), too.
 
-With a C-u prefix, uses errortrace for improved stack traces.
-Otherwise follows the [`racket-error-context`](#racket-error-context) setting.
+If your source file has a syntax or runtime error, a "skeleton"
+of your file is evaluated to get identifiers from module
+languages, `require`s, and definitions. That way, things like
+completion and [`racket-describe`](#racket-describe) are more likely to work while
+you edit the file to fix the error. If not even the "skeleton"
+evaluation succeeds, you'll have only identifiers provided by
+racket/base, until you fix the error and run again.
 
 Output in the `*Racket REPL*` buffer that describes a file and
-position is automatically "linkified". To visit, move point
-there and press <kdb>RET</kbd>, mouse click, or use a
-Compilation mode command such as C-x ` (next error).
-Examples of such text include:
+position is automatically "linkified". Examples of such text
+include:
 
 - Racket error messages.
 - `rackunit` test failure location messages.
 - `print`s of `#<path>` objects.
 
+To visit these locations, move point there and press RET or mouse
+click. Or, use the standard `next-error` and `previous-error`
+commands.
+
 In the `*Racket REPL*` buffer you can issue some special
 commands. Some of them are the foundation for Emacs commands.
 Others are available only as a command in the REPL.
@@ -103,13 +113,13 @@ Major mode for results of [`racket-profile`](#racket-profile).
 key             binding
 ---             -------
 
-RET             racket--profile-visit
-,               racket--profile-sort
-g               racket--profile-refresh
-n               racket--profile-next
-p               racket--profile-prev
-q               racket--profile-quit
-z               racket--profile-show-zero
+RET		racket--profile-visit
+,		racket--profile-sort
+g		racket--profile-refresh
+n		racket--profile-next
+p		racket--profile-prev
+q		racket--profile-quit
+z		racket--profile-show-zero
 
 
 ```
@@ -145,15 +155,15 @@ For more information see:
 key             binding
 ---             -------
 
-C-c             Prefix Command
-g               racket-logger-clear
-l               racket-logger-topic-level
-n               racket-logger-next-item
-p               racket-logger-previous-item
-w               toggle-truncate-lines
-x               racket-logger-exit
+C-c		Prefix Command
+g		racket-logger-clear
+l		racket-logger-topic-level
+n		racket-logger-next-item
+p		racket-logger-previous-item
+w		toggle-truncate-lines
+x		racket-logger-exit
 
-C-c C-z         racket-repl
+C-c C-z		racket-repl
 
 
 ```
@@ -534,16 +544,16 @@ special commands to navigate among the definition and its uses.
 key             binding
 ---             -------
 
-TAB             racket-check-syntax-mode-goto-next-def
-.               racket-check-syntax-mode-goto-def
-h               racket-check-syntax-mode-help
-j               racket-check-syntax-mode-goto-next-def
-k               racket-check-syntax-mode-goto-prev-def
-n               racket-check-syntax-mode-goto-next-use
-p               racket-check-syntax-mode-goto-prev-use
-q               racket-check-syntax-mode-quit
-r               racket-check-syntax-mode-rename
-<backtab>       racket-check-syntax-mode-goto-prev-def
+TAB		racket-check-syntax-mode-goto-next-def
+.		racket-check-syntax-mode-goto-def
+h		racket-check-syntax-mode-help
+j		racket-check-syntax-mode-goto-next-def
+k		racket-check-syntax-mode-goto-prev-def
+n		racket-check-syntax-mode-goto-next-use
+p		racket-check-syntax-mode-goto-prev-use
+q		racket-check-syntax-mode-quit
+r		racket-check-syntax-mode-rename
+<backtab>	racket-check-syntax-mode-goto-prev-def
 
 
 ```
@@ -592,7 +602,7 @@ can turn it off by setting `input-method-highlight-flag` to nil via
 `M-x customize-variable`.
 
 ### racket-align
-<kbd>M-]</kbd>
+<kbd>M-x racket-align</kbd>
 
 Align values in the same column.
 
@@ -652,7 +662,7 @@ usual. For example:
 See also: [`racket-unalign`](#racket-unalign).
 
 ### racket-unalign
-<kbd>M-}</kbd>
+<kbd>M-x racket-unalign</kbd>
 
 The opposite of [`racket-align`](#racket-align).
 
@@ -841,11 +851,6 @@ Face for self-evaluating expressions like numbers, symbols, strings.
 ### racket-here-string-face
 Face for self-evaluating expressions like numbers, symbols, strings.
 
-### racket-paren-face
-No documentation.
-
-
-
 ### racket-check-syntax-def-face
 Face [`racket-check-syntax`](#racket-check-syntax) uses to highlight definitions.
 

error.rkt

@@ -67,13 +67,15 @@
                  (context-item->string x))
                "\n"))
 
-(define-runtime-path run.rkt "run.rkt")
+(define-runtime-path run.rkt       "run.rkt")
+(define-runtime-path namespace.rkt "namespace.rkt")
 (define (system-context? ci)
   (match-define (cons id src) ci)
   (or (not src)
       (let ([src (srcloc-source src)])
         (and (path? src)
              (or (equal? src run.rkt)
+                 (equal? src namespace.rkt)
                  (under-system-path? src))))))
 
 (define (under-system-path? path)