Throw joined errors if error is empty on commit

Fixes tpope/vim-fugitive#907

plugin/fugitive.vim

@@ -1121,7 +1121,7 @@ function! s:Commit(args, ...) abort
       elseif error ==# '!'
         return s:Status()
       else
-        call s:throw(error)
+        call s:throw(empty(error)?join(errors, ' '):error)
       endif
     endif
   catch /^fugitive:/