Stop swallowing exceptions in log_prob method

[WardBrian]

Submisison Checklist

• Run tests: ./runCmdStanTests.py src/test
• Declare copyright holder and open-source license: see below

Summary:

If the model throws an exception, currently log_prob gives no output to indicate this.

Intended Effect:

The exception message is now printed.

How to Verify:

Side Effects:

Documentation:

Copyright and Licensing

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By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[mitzimorris]

LGTM - and we should have been doing this all along.