Add error handling and watchdog messages

Just adding a watchdog message when a request error is returned.

Module will still return NULL where there is an error returned.

To check for errors you will need to pass the variable with the "response" to a rules component. In the rules component you need to create a condition to check for a zero-length string. This can be done using a regular expression.

Why not just add another provided variable, like error_code and/or error_message?