Feature request: More accurate exception types

[andy-esch]

Right now all exceptions are CartoException, which makes it hard to handle specific exceptions but let others be raised. E.g., If I want to print a special message about limits being hit, I would have to use the error string which could change without notice in future updates to the platform.

[andy-esch]

Instead of CartoException, what about returning the requests' exceptions instead? http://docs.python-requests.org/en/master/_modules/requests/exceptions/