AM web service V2: Refactoring & optimization

Make error handling better

• Currently on error the function returns null, and the callee checks for null value which is not the best approach.
• Guess the best is to modify the exception to give a meaning full message, and then re-throw the same. A base try..catch block can be put in to handle all exceptions.
• Gracefully fail invalid ids - Server shouldn't return error even if ids are invalid.