Help RP's understand actionable exceptions from create() and get()

[MasterKale]

This PR attempts to pull together any exceptions raised by create() and get() to help RP's understand what exceptions may be encountered when using WebAuthn. The intention here is to help RP's understand which errors might be surfaced to the user, and which should not.

Addresses #1859.

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[MasterKale]

I've finally cobbled together reasons for all of the exceptions during both registration and authentication.

...Except I cop out a bit with NotAllowedError because it has many more possible reasons it gets raised, and in practice clients have overloaded this exception with causes not documented in the spec. I thought it prudent to present this error as one that RPs should prepare to handle as a general, "the user canceled the ceremony, or something went wrong" exception and handle it as such. This is as opposed to encouraging each RP to try and interpret all possible reasons the issue was raised. I'm open to feedback on this approach.

[herrjemand]

100% support this @MasterKale. Recently working on client lib, NotAllowedError is kinda useless. *)