Make file edit error message less specific

The truth is that at the point where there error message is written we
can only guess what went wrong.

app/services/files/update_service.rb

@@ -33,7 +33,7 @@ module Files
       if created_successfully
         success
       else
-        error("Your changes could not be committed, because the file has been changed")
+        error("Your changes could not be committed. Maybe the file changed, or there was nothing to commit?")
       end
     end
   end