RB.apiCall has had a long-existing bug where built-in callbacks could

get shadowed by options arguments. The intent of the implementation is

that the built-in error and complete callbacks always run, and will

then call any ones which are passed in by the caller. Unfortunately, the

order of arguments passed to $.extend() prevented this from working as

intended. For existing versions of Review Board, the only noticeable

effect was the error shadows within the OAuth UI, which were highly

unlikely to occur. With the JS refactoring that happened for 5.0, we

added a complete callback for diff fragments, which override the

built-in one, causing the activity indicator to never disappear.
To fix this, I've rearranged the $.extend() call a bit in order to

ensure that the built-in callbacks always run first. Because the

built-in error callback has its own display mechanism, I've just removed

the error display from the OAuth code.

Ran js-tests.
Verified that the diff fragment loader now correctly hid the activity

  indicator.
Checked that the OAuth error handlers worked as expected.
Audited all callsites to RB.apiCall to ensure that there wouldn't be

  other issues.