More accurate error message.

autoload/diff.vim

@@ -40,7 +40,7 @@ function! diff#run_diff(realtime, use_external_grep)
   if v:shell_error
     " A shell error indicates the file is not tracked by git (unless something
     " bizarre is going on).
-    throw 'git-diff failed'
+    throw 'diff failed'
   endif
 
   return diff

plugin/gitgutter.vim

@@ -76,7 +76,7 @@ function! GitGutter(file, realtime)
 
         call utility#save_last_seen_change(a:file)
       endif
-    catch /git-diff failed/
+    catch /diff failed/
       call hunk#reset()
     endtry
   else