Add CyIpoptEvaluationError and handle in callbacks

[Robbybp]

Fixes #210

• New cyipopt.exceptions module that contains only CyIpoptEvaluationError for now
• In function and derivative callbacks, if CyIpoptEvaluationError is raised by user's callback, we ignore the exception and return false to Ipopt
• Test this behavior on HS071. Ipopt completes the solve (by cutting the step size) if the error occurs in the objective or constraints, but quits immediately if the error occurs in the derivatives.

The tests are integration tests, which I've included in test_hs071.py. I can move these to their own module if that would be more appropriate.

[moorepants]

Thanks for this. LGTM.