If any errors are encountered rendering diagrams in the user manual, exit with an error code

Error messages for all diagrams are printed to STDERR, before xml2hs exits.
Shake exits after xml2hs fails, without attempting later build stages.
Merge pull request #28 from diagrams/travis-error-on-failed-diagrams

If any errors are encountered rendering diagrams in the user manual,
exit with an error code.

Closes #20.