When diffs encounter loading errors (such as 404s or 500s in the diff
request), the diff would formerly bubble up an event that showed an
error message that blocks the entire page. This works well in the diff
view because the singular diff is the main content of the page.
However, when expanding many files inline in the change view, if only
some of the diffs encounter loading errors, the error message need not
obstruct the rest of the page, which may contain diffs that load fine.
With this change, inline diffs show their own descriptive loading
errors, and won't hide the rest of the content. The structure of the
diff loading request is refactored so that errors can be caught and
handled cleanly in the promise chain and the overall async-loop that
renders many diffs in order will not be halted when one diff fails.
Bug: Issue 9590
Change-Id: I9d984d39dcec90119d05acd94afad938e5fb7c1e
eb04344f67