Handle more import update edge cases

This PR handles some import update edge cases:

• reject updates with invalid review action ids
• reject merge review actions that don't result in modifications to the underlying contact (i.e. all attributes are keep_existing)
• when updating with a non-merge action type, the system should remove merge directive info from the review action
• reject merge directives when the attribute key doesn't exist (should be enforced by Pydantic but this adds a test to make sure)

This PR also goes through and reformats the import error case tests to ensure that the errors under test are correct, and there aren't any lingering errors that may cause false test failures. Each test now must define a passing case derived from the failing test, with the error explicitly corrected in the payload.