Better management for StatusEntry an related errors

[ugoamanoh]

accept and use an optional StatusEntry param for checkStatus() to deduplicate parsing of credentials and to support verification of credentials that don't necessarily embed a status entry.

rethink exceptional cases to accommodate credentials that don't embed status, and create more informative errors for misconfigured status methods or malformed entries.

create and use a CredentialStatus implementation that is only used to signal a credential that doesn't declare a status checking method.

[ugoamanoh]

Implementation Notes

• return DIDDocument in JWTTools().verify()

• checkStatus method to allow an optional StatusEntry parameter

• if StatusEntry is not provided then parse from credential

• create new CredentialStatus impl call StatusError which can handle error messages

• if no StatusEntry is supplied or in the credential return StatusError retun an error