Fix image proxy get error handling

[sarayourfriend]

Fixes

Fixes #3454 by @sarayourfriend

Also fixes #3276 because I really need to stop fixing this every time I spin the app up locally after a week of not running it.

Description

Replaces the error handling code specific to requests exceptions with aiohttp handling. Just something I missed in code and we missed in reviewing #3388.

Testing Instructions

Review the code changes and confirm I've covered the cases. I had to make a change to one of the error keys but I'll explain it in a comment on that line in the PR.

Tests and CI should pass, of course!

Checklist

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[sarayourfriend]

The response text is ambiguous from aiohttp (I wasn't entirely sure what it was and whether it matched wherever requests as using). I removed it because I figure we really care more about the status code, and if we need specific details about the error itself, we can look at the logs for the exc.message or review the captured exception in sentry (if relevant).

[sarayourfriend]

This and the other changes in this test module are cleanup. Using pook.on and pook.off like this means that if api_client.get results in an exception (which is a test failure), then pook would never get turned off. It's exactly what a context manager is for anyway.

[sarayourfriend]

These changes are for #3276

[obulat]

Changes work well. Nice cleaning up of the code and tests!

[stacimc]

LGTM!