MayaScene bindings now catch boost python exceptions

I ran into a case where some custom attribute readers were erroring in caribou and crashing maya. Turns out this is because they were raising a boost::python::error_already_set, which wasn't getting caught by anything, and triggered another uncaught exception. I've now put try/catch blocks in the CustomTagReader and CustomAttributeReader to avoid this situation in the future.
Merge pull request #301 from davidsminor/mayaScenePythonExceptions

MayaScene bindings now catch boost python exceptions