ENH Gracefully handle AJAX failures

[emteknetnz]

Issue #110

I tested by adding the following in to controller endpoints on LinkFieldController:

• $this->jsonError(500, 'FFF'); - expected failure
• throw new Exception('AAA'); - unexpected failure
• set_time_limit(1);while(true); - simulate timeout

I tested when:

• Reading links LinkFieldController::getLinkData()
• Deleting links LinkFieldController::linkDelete()
• Saving links LinkFieldController::save()

[GuySartorelli]

There are conflicts to resolve.

[emteknetnz]

Conflicts resolved

[GuySartorelli]

LGTM.

In a perfect world we'd add some JEST tests for this but I'm not too bothered about it, the main thing IMO is that the data doesn't end up in a bad state which it looks like it would be pretty hard to accomplish via thrown exceptions etc.