Refactored ClosureAPI::Request#check_error_code

Around 30 lines of code were bought down to only about 6. The original
basically had a long `case` statement testing the `@error_code` instance
variable for different error codes, and then raising a very similar
error to other cases. I've put all the possible exceptions and all
possible error codes in an array and then used the `zip` method to pair
them together. Then all I did is raised couple of exceptions with
helpful messages containing the error codes. Pretty much the same
functionality as the original `case` statement, but a lot shorter.