A way to fail on missing docs

[NorfairKing]

I'd like to ensure that my library is fully documented by failing the build if haddock fails. So I think I'd like something like -Werror for haddock, because I see that haddock can already warn against missing docs.

[infinisil]

I just thought of a slighty hacky way to achieve this: Since there's the --no-warnings flag, it's possible to do one run without the flag and one with, if the output changes based on the flag, there's warnings.

[NorfairKing]

cc @tchoutri