Handle errors when uploading key signatures

[jryans]

We currently do not check for errors when uploading key signatures (JS SDK method uploadKeySignatures). This is probably because the failures are returned inline in the response for each signature, rather than in the top-level HTTP response code.

We should track these errors and communicate failure to the user somehow.

[nadonomy]

When does this happen specifically in the user journey? Does it happen when bootstrapping? (as I see it was referenced in #11212). Can it happen in any other instances?

[t3chguy]

[nadonomy]

I think given the failure mode is that users can't complete their goal— out of pragmatism for now I'd throw up an error message in a modal, where they can hit OK and start doing whatever they were attempting again.

If this turns out to be a very common failure mode we could polish this into much nicer contextual error feedback.

[foldleft]

Reference: https://github.com/matrix-org/matrix-doc/blob/master/proposals/1756-cross-signing.md