When receiving an unexpected error from the server, the frontend code for the

pre-commit upload would hit a TypeError, preventing it from completing the

process and leaving the browser with a forever "Loading..." message. This

change updates it to check when the response can't be parsed as JSON and show

"Unknown error" instead.
While I was in here I fixed up some of the nearby localization to be more

correct.

Checked that uploading a diff through the pre-commit UI still worked as

  expected.
Ran js-tests.