Add robust exceptions to Python client

[jochasinga]

Currently the Python client does not have a robust and friendly exception classes and throws a Segmentation fault error on the erratically formatted schema files and this error, which does not do a good job at hinting what goes wrong:

terminate called after throwing an instance of 'std::runtime_error'
  what():  Not a number.
Aborted (core dumped)

We should add some important exception classes to the client with more information that handle the lower-level errors bubbling up to the caller.