Avoid ignoring LaTeX export output errors

* lisp/org-macs.el (org-compile-file): When PROCESS is a list, keep
output of all the listed commands, not just the last one.
* lisp/ox-latex.el (org-latex-pdf-process): Clarify that the process
output is parsed to detect compilation errors/warnings.

lisp/org-macs.el

@@ -315,8 +315,13 @@ it for output."
 			       (?F . ,(shell-quote-argument full-name))
 			       (?o . ,(shell-quote-argument out-dir))
 			       (?O . ,(shell-quote-argument output))))))
-	   (dolist (command process)
-	     (shell-command (format-spec command spec) log-buf))
+           ;; Combine output of all commands in PROCESS.
+           (with-current-buffer log-buf
+             (let (buffer-read-only)
+               (erase-buffer)))
+           (let ((shell-command-dont-erase-buffer t))
+	     (dolist (command process)
+	       (shell-command (format-spec command spec) log-buf)))
 	   (when log-buf (with-current-buffer log-buf (compilation-mode)))))
 	(_ (error "No valid command to process %S%s" source err-msg))))
     ;; Check for process failure.  Output file is expected to be

lisp/ox-latex.el

@@ -1399,6 +1399,9 @@ A better approach is to use a compiler suit such as `latexmk'."
       "%latex -interaction nonstopmode -output-directory %o %f"))
   "Commands to process a LaTeX file to a PDF file.
 
+The command output will be parsed to extract compilation errors and
+warnings according to `org-latex-known-warnings'.
+
 This is a list of strings, each of them will be given to the
 shell as a command.  %f in the command will be replaced by the
 relative file name, %F by the absolute file name, %b by the file