ignore stale `users/show` responses - fixes #741

What does this PR do?

Simplest fix: if a response comes in that does not match the current username, ignore it. Since the username changed, another request has been sent already, and the response will arrive at some point.

The "in case of error" code is a bit annoying because misskeyApi doesn't pass us any detail apart from the error code, so we now close over the username we requested, and compare against that.

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requested review from @fEmber, @puppygirlhornypost, and @Marie

approved this merge request

approved this merge request

enabled an automatic merge when all merge checks for bf6ca8ef pass

merged

mentioned in commit 669e5c6c