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Fixed promise error handling

In many places where we use Promises (e.g. for making fetch requests to
our backend APIs) we were previously incorrectly handling promise
rejection. We were often doing `promise.then(fn).catch(fn)`, which is not
a great idea - then the `.catch` handler catches any errors in the
Promise AND any errors in the `.then` handler. Instead, we should prefer
to do `promise.then(resolve, reject)` - this makes the `reject` handler
narrower in it's responsibility.

The main problem with the former pattern is that the `.catch` handler
can swallow any exceptions which occur in the `.then` handler, and we
will not see them. This can lead to strange and unhelpful stack traces
(such as the react `getHostNode` error that crops up sometimes).

Anyway, this PR should fix it! I also simplified and streamlined our
`fetchJSONWithCSRF` function while I was at it.

pr #3022

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