Should this library throw errors from the `did:key` spec?

[aljones15]

The did:key spec now contains multiple normative statements about did:key. While we do throw many errors related to those statements, we don't throw them in a way that conforms to the spec.

I'm proposing that checks that can be made at the general level of any did:key be made here and then checks related to key material be made in their respective libraries.

That means the following checks need to be made in this library:

• MUST raise invalidDid error if scheme is not did (NOTE: this check might be needed elsewhere)
• MUST raise invalidDid error if method is not key (this check is did:key specific)
• MUST raise invalidDid if version is not convertible to a positive integer value.
• MUST raise invalidDid if the multibaseValue does not begin with the letter z.

The errors might be specific to the didResolver:

• If "didDocument.id" is not a valid DID, an invalidDid error MUST be raised
• If verificationMethod.id is not a valid DID URL, an invalidDidUrl error MUST be raised.
• For Signature Verification Methods, if options.enableExperimentalPublicKeyTypes is set to false and publicKeyFormat is not Multikey, JsonWebKey2020, or Ed25519VerificationKey2020, an invalidPublicKeyType error MUST be raised.
• For Encryption Verification Methods, if options.enableExperimentalPublicKeyTypes is set to false and publicKeyFormat is not Multikey, JsonWebKey2020, or X25519KeyAgreementKey2020, an invalidPublicKeyType error MUST be raised.
• If verificationMethod.controller is not a valid DID, an invalidDid error MUST be raised.

We can throw an error like this:

export class InvalidDid extends Error {
  constructor(message) {
    super(message);
    this.name = 'InvalidDidError';
    this.code = 'invalidDid';
  }
}

export class InvalidDidUrl extends Error {
  constructor(message) {
    super(message);
    this.name = 'InvalidDidUrlError';
    this.code = 'invalidDidUrl';
  }
}

Related:
digitalbazaar/ed25519-verification-key-2020#18
w3c-ccg/did-key-spec#60