Propagate exclude credential

[iinuwa]

Fixes#211.

This is going to conflict with #204 though, so I'll wait until that lands to merge this.

Fixes#214 also.

[msirringhaus]

lgtm!

[iinuwa]

Rebased, so this is a bit smaller now. I tested and this works to return to the error to the caller, but there's a follow-up needed from #204 to prevent sending a ErrorCancelled error right after the CredentialExcluded to the UI, so the window flashes from "This authenticator is already registered" to "Something went wrong."

Still need to add tests before merging

[iinuwa]

I modified the tests introduced to allow modeling a handler that can send events after receiving cancellation, which caused some tests to fail, and then made the *Stream structs aware of the cancellation token to fix it. Now Credential Excluded error shows up in the UI correctly.

I'm going to go ahead and merge this and start working on tagging 0.3.