Warn on stylus errors

Fixes #51. Alternative to #52 & #53 requested by @shama.

Log out stylus errors without exiting, and without an error code.  Not sure I like this.  I'm using grunt-contrib-stylus as part of the build script on a continuous deployment server.  It's a little scary that a deploy could go succeed when styles failed to build...

@shama @cowboy, are there other strategies for failing the build on stylus errors?  Parse the grunt output and look for errors?