Add tests for error cases in did-resolver.spec.ts

This commit adds new tests in did-resolver.spec.ts to cover some of the previously uncovered error-related branching paths.

This commit contributes partly to issue decentralized-identity#136.

Based on the output of `npm run test:node` before vs. after this commit, the `% Branch` is now increased by 9.66 percentage points, and the `% Stmt` and `% Lines` by 3.29 percentage points.

Before:
```
did-resolver.ts |   87.91 |    78.57 |   66.66 |   87.91
```

After:
```
did-resolver.ts |   92.34 |    94.11 |   66.66 |   92.34 |
```

src/core/dwn-error.ts

@@ -15,6 +15,9 @@ export class DwnError extends Error {
 export enum DwnErrorCode {
   AuthenticateJwsMissing = 'AuthenticateJwsMissing',
   AuthorizationUnknownAuthor = 'AuthorizationUnknownAuthor',
+  DidMethodNotSupported = 'DidMethodNotSupported',
+  DidNotValid = 'DidNotValid',
+  DidResolutionFailed = 'DidResolutionFailed',
   GeneralJwsVerifierInvalidSignature = 'GeneralJwsVerifierInvalidSignature',
   GrantAuthorizationGrantExpired = 'GrantAuthorizationGrantExpired',
   GrantAuthorizationGrantMissing = 'GrantAuthorizationGrantMissing',