Provide more diagnostics and feedback esp. w.r.t. MATLAB input

[mhucka]

Need to provide much more feedback to users when the input does not match expectations, or uses matlab features that are not supported, or something else goes wrong. Currently, the converter does not provide much feedback about unsupported constructs beyond just saying that conversion failed. We need to make the system provide more feedback so the user understands why it failed.

[mhucka]

Subtasks:

• complain if input has multiple files
• complain if input has things like .mat files

[mhucka]

More things to do: test bad inputs -- what kind of errors do you get? improve warnings when it can't convert formulas to AST & others -- create set of invalid_**.m tests and use py.test assert to test failures