More user friendly error message

[josiahjohnston]

Make an error message more user friendly for people who have errors in their custom scoring functions.

What does this PR do?

Give the user another hint as to what may have caused an error.

Where should the reviewer start?

Look at the diff

How should this PR be tested?

Standard tests

Any background context you want to provide?

#983

It would be nice to have an debugging option to make tpot.gp_deap:_wrapped_cross_val_score() print exceptions rather than hiding all exception. It would probably be best to control that via by the verbosity setting, but a separate hacky flag like tpot.gp_deap.display_exceptions=True would be adequate for local debugging purposes.

What are the relevant issues?

#983

Screenshots (if appropriate)

Questions:

• Do the docs need to be updated?
  No
• Does this PR add new (Python) dependencies?
  No

[coveralls]

Coverage remained the same at 96.747% when pulling cf26495 on josiahjohnston:user-friendly-error-message into fc26b71 on EpistasisLab:development.

[weixuanfu]

Thank you for the PR and fix. I will merge it to dev branch.