ApiSandbox: Fix HTTP error handling

ApiSandbox: Fix HTTP error handling

Since bf69459, ApiSandbox seems to stall out when an API request
results in an HTTP error (e.g. due to a PHP fatal error). Before that
revision, it displayed the 'apisandbox-results-error' message in this
situation.

Apparently the jQuery 3 changes to Deferred behavior caused it to be
impossible to have a then() filter return `this` (or anything else) in
order to avoid replacing the existing promise that's being resolved or
rejected.

Bug: T165857
Change-Id: I3f646cdfe7fe8987437980790788821f51e728d1