Overhaul thrown exceptions

[unfulvio-godaddy]

Summary

Adds a WP_MockException object and overhauls exceptions used or referenced within project.

After consideration, I decided to go for more specific exceptions in the PHP stdlib - such as RunTimeException or InvalidArgumentException when applicable. I realized that WP_Mock already uses other domain-specific exceptions when it comes to assertions failed or mockery exceptions, so perhaps at the moment we don't need a specific WP_Mock exception.

Had to make additional changes due to PhpStan errors after updating code.

Closes: #189

Contributor checklist

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly
• I have added tests to cover changes introduced by this pull request
• All new and existing tests pass

Testing

• Tests and code analysis passes

Reviewer checklist

• Code changes review
• Documentation changes review
• Unit tests pass
• Static analysis passes

[coveralls]

Coverage: 11.869% (+0.7%) from 11.201% when pulling fe8e5c3 on issue-189/introduce-wp-mock-exception into cdac012 on trunk.

[agibson-godaddy]

Approving this because it seems fine, but I did leave a couple comments for you to consider.

[unfulvio-godaddy]

@agibson-godaddy sorry I went to a different direction after reading your comments but mostly re-evaluating the purpose of this PR - in the end maybe WP_Mock is already using some specific exceptions depending on context (from PhpUnit expectation failed exception, mockery exceptions, etc.). I have added a couple of InvalidArgumentException or RunTimeException where it seemed to make most sense. Perhaps there is no need to use a generic exception at the moment.