Improve error messages if Python fails validation

[mwaddoups]

Right now we just pass back any Python tracebacks as is, these aren't very helpful for the end user.

We can change this so that we

• Provide informative error messages in Python for use cases we've identified.
• Generate a "contact us" message for any error which is not identified.

[mwaddoups]

The error messages are now more informative - they just display a single line.