Don't error out when vim-shell is not installed (issue #7)

This is the most obscure interaction of syntax and semantics I
have found in the 5+ years I've been writing Vim script now :-(

For more information, see issue #7 on GitHub:
  #7

README.md

@@ -38,7 +38,7 @@ from the source code of the miscellaneous scripts using the Python module
 <!-- Start of generated documentation -->
 
 The documentation of the 80 functions below was extracted from
-15 Vim scripts on June 26, 2013 at 00:09.
+15 Vim scripts on June 26, 2013 at 10:55.
 
 ### Handling of special buffers
 

autoload/xolox/misc.vim

@@ -1,7 +1,7 @@
 " The version of my miscellaneous scripts.
 "
 " Author: Peter Odding <peter@peterodding.com>
-" Last Change: June 25, 2013
+" Last Change: June 26, 2013
 " URL: http://peterodding.com/code/vim/misc/
 
-let g:xolox#misc#version = '1.8.3'
+let g:xolox#misc#version = '1.8.4'

autoload/xolox/misc/os.vim

@@ -1,7 +1,7 @@
 " Operating system interfaces.
 "
 " Author: Peter Odding <peter@peterodding.com>
-" Last Change: June 25, 2013
+" Last Change: June , 2013
 " URL: http://peterodding.com/code/vim/misc/
 
 function! xolox#misc#os#is_mac() " {{{1
@@ -250,11 +250,13 @@ function! xolox#misc#os#can_use_dll() " {{{1
   " and c) the compiled DLL included in vim-shell works, we can use the
   " vim-shell plug-in to execute external commands! Returns 1 (true)
   " if we can use the DLL, 0 (false) otherwise.
+  let can_use_dll = 0
   try
-    return xolox#shell#can_use_dll()
+    let can_use_dll = xolox#shell#can_use_dll()
   catch /^Vim\%((\a\+)\)\=:E117/
-    return 0
+    " Silence E117.
   endtry
+  return can_use_dll
 endfunction
 
 function! s:readfile(fname, label, cmd) " {{{1

doc/misc.txt

@@ -151,7 +151,7 @@ from the source code of the miscellaneous scripts using the Python module
 'vimdoctool.py' included in vim-tools [5].
 
 The documentation of the 80 functions below was extracted from 15 Vim scripts
-on June 26, 2013 at 00:09.
+on June 26, 2013 at 10:55.
 
 -------------------------------------------------------------------------------
                                              *misc-handling-of-special-buffers*