Raise error when Transform class can not be found inside girder_worker

[matthewma7]

I wrote a custom Transform class, but it was put inside a girder plugin so when serialized to girder_worker the dynamic python module name controlled by girder is probably not understandable by girder_worker. As a result, girder_worker wasn't able to deserialize the class and call the transform(), so the transform failed.
However, girder_worker didn't raise any error message regarding the failure to find Transform class. If an error message was raised it would help me to diagnose my problem.

[jbeezley]

Related to #219.