Tracking issue: Clearer, more effective error messages

[obi1kenobi]

We should aim to always be improving the quality of our error messages.

Here are outstanding issues with them:

• [ ] Errors don't currently include span information. This is a big lift and will require plumbing that information throughout the frontend.
• [ ] When a query fails to validate against the schema, .unwrap() on the Result produces a Debug-style printout like CompilationError(ValidationError(NonExistentPath(["CrateDiff", "baseline", "item", "Enum", "variant", "StructVariant", "field"]))). This is not ideal:
  • [ ] it should clearly mention the problem is that the query didn't match the schema
  • [ ] it should ideally include Display output rather than Debug output
  • [ ] perhaps the naming can be better as well?