So I don't forget, I'm adding some of my ideas/suggestions for improving errors here. Let me know if there is a better place for this.

Make error messages more consistent. Especially around how {field}s are displayed.

• statement-data-missing-field message
  • Mandatory field missing in the statement data: {field} -> Mandatory field '{field}' missing in the statement data
• statement-data-invalid-field message
  • Invalid input for {field} -> Invalid input for field '{field}' or Invalid value for field '{field}'
• patched-statement-missing-field message
  • Mandatory field missing in the patched statement: {field} -> Mandatory field '{field}' missing in the patched statement
• patched-statement-invalid-field message
  • Invalid input for '{field}' in the patched statement -> Invalid value for field '{field}' in the patched statement

Make json-patch error codes more consistent

• Be consistent with other "invalid" error messages (e.g. invalid-item-id):
  • invalid-patch-operation
  • invalid-patch-field-type
  • missing-json-patch-field -> invalid-patch-missing-field
• or be consistent with statement-data-missing-field and statement-data-invalid-field:
  • invalid-patch-operation -> json-patch-invalid-operation (or json-patch-invalid-op)
  • invalid-patch-field-type -> json-patch-invalid-field-type
  • missing-json-patch-field -> json-patch-missing-field

Make error messages less verbose

• patch-test-failed
  • Test operation in the provided patch failed. At path '{path}' expected '{json_encoded_value}', actual: '{json_encoded_actual}' could instead be one of:
    • -> Test operation failed at path '{path}'. Expected '{value}' to equal '{actual}'.
    • -> Test operation failed. Expected '{value}' to equal '{actual}' at path '{path}'.
    • -> Failed to assert '{value}' equals '{actual} at path '{path}' - do we need to repeat the error code and say "Test operation failed"?
    • -> any option above but just '{path}' instead of path '{path}'?

Add context to more error responses?
Not all error responses have a context field providing additional machine readable information about the error

Harm prevention mechanisms result in an unexpected error
We should return a specific error responses instead of an unexpected error

REST API error responses are inconsistent with Mediawiki error responses
See T313721