make DidResolutionResult.DidDocument not nullable

[mistermoe]

Concretely, DidResolutionResult.didDocument should no longer be nullable, and instead of an error being buried in didResolutionMetadata per the did core spec, we need to throw a DidResolutionException that itself has a didResolutionMetadata member.

This way all errors thrown by didResolver.resolve can be handled in a single code path vs try/catching and checking if resolutionResult.hasError() and didDocument being nullable.