changes to canvas interaction layer to better reflect the skd's use of e...

...xceptions
I don't think many libraries treat their exception classes as constants like you are here.  I would expect you to instantiate a new instance of a custom exception class with at the very least some type of message body.  That being said, the exceptions that the SDK throws (through the Requests library) are much more informative than what you have here, so I wonder why you'd even bother to throw your own?  This is another thing to consider after our conversation around your exception hierarchy.