If any errors are encountered rendering diagrams in the user manual, exi...

...t 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.

This addresses #20.  I'd appreciate another pair of eyes.  Shake exits with code 1; travis docs say:

[T]he script is that it should use an exit code 0 on success, any thing else is considered a build failure.  (http://about.travis-ci.org/docs/user/build-configuration/)

Practically, would it be better if Shake tried to keep going, in order to expose any other errors?

Aesthetically, would it be better to move all the putStrLn calls (in error cases) into HXT's error infrastructure?  I haven't found the right arrow to do this yet, but it must be there, right?

I've only tested on my machine---didn't want to push a broken rst file to travis just to show that it errors.