Initial error handling

[andrii-otroshko-forte]

This PR contains initial error handling in the Fauna client:

• Implemented the required error types as C# exceptions.
• Updated the client to handle these errors according to the proposed hierarchy.
• Added handling for HttpClient errors, converting them into Fauna exceptions with original errors as inner exceptions.
• Updated related Client unit tests. If this implementation does not require refactoring, I plan to add more tests for extensive coverage

[pnwpedro]

Good start, and the direction is right. I added a few specific comments, but generally what I'd like to see is more specificity on each type. Ideally minimize the number of nullable properties so users can confidently interact with error objects without reading documentation about what might be null. In addition, make sure they are elevated to the top level.

[pnwpedro]

A few more things to sort out I think