Improve client error messages

[cycraig]

Description of change

Throw a more informative DIDNotFound error message when the network of the DID does not match that of the Client when resolving it.

Removes several defunct and unused error variants from identity-iota.

Links to any relevant issues

Fixes #505

Type of change

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• Bug fix/chore (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Tests and examples pass locally.

Change checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes