Optionally return non-zero in case of doc issues

[blueyed]

trafficstars

It would be useful if ldoc could exit with a non-zero return code in case of errors like param and formal argument name mismatch.

This could be used in CI builds to make sure the documentation is correct.

[stevedonovan]

--fatalwarnings already existed for exactly this purpose, but it didn't cover parser errors. So this fix should be about right now.

[alerque]

Fixed in ebdbf7d.