Fixes #7554 - Show user friendly error message on 413

This is kind of tricky to test. You need to run the web server with passenger (no apache proxy either). Then just upload a file 10MB+. You should get "Error during upload: undefined" without this change. Some options:

1. Set up a nightly install. Turn off asset compilation and apply the patch.
2. Setup your dev install to use passenger
3. Hack the action to return the text you see in the js test.