Move CallCancelled from core and all RPC constants into an enum

[lukaszgo1]

Keeping as a draft until development cycle for 2022.1 starts.

Link to issue number:

None

Summary of the issue:

Currently various RPC constants we use are scattered through out the code base. Also some stuff is defined in core but is not related to core in any way.

Description of how this pull request fixes the issue:

• CallCancelled is moved to the new exceptions module
• All RPC constants both from core and from logHandler are moved into an enum .

Testing strategy:

Made sure that logging still works and that CallCancelled is raised when appropriate.

Known issues with pull request:

This breaks backwards compatibility - while these moves are nice from the code layout perspective I don't think they are important enough to justify providing backwards compat.

Change log entries:

For Developers

• core.CallCancelled becomes exceptions.CallCancelled
• All constants starting with RPC from core and logHandler are moved into RPCConstants.RPC enum.

Code Review Checklist:

• Pull Request description:
  • description is up to date
  • change log entries
• Testing:
  • Unit tests
  • System (end to end) tests
  • Manual testing
• API is compatible with existing add-ons.
• Documentation:
  • User Documentation
  • Developer / Technical Documentation
  • Context sensitive help for GUI changes
• UX of all users considered:
  • Speech
  • Braille
  • Low Vision
  • Different web browsers
  • Localization in other languages / culture than English

[lukaszgo1]

Given development cycle for 2022.1 has began I'm marking this as ready for review.