When the parameter testPlanKey is invalid, API should return a 400 Bad Request instead of a 503 Service Unavailable

Short Description: When the parameter testPlanKey is invalid, API should return a 400 Bad Request instead of a 503 Service Unavailable.

Steps to Reproduce:
1 - Create a POST request to the following endpoint 'url'/api/v1/import/execution/robot
2 - In the parameters enter a testPlanKey that does not exist and send the request

Actual Result: A 503 error Service Unavailable is displayed with the error message "Server Error - Error processing JQL batch search results - 400".
Expected Result: A 400 error Bad Request should be displayed with the error message "Error importing results: issue with key X does not exist or is not of issue type Test Plan!"