Add test for emiting jade's error.
Don't double wrap jade template errors

Makes debugging template errors much simpler. With
a double wrap, stacktrace will be truncated such
that it looks like something is wrong with
gulp-jade, when it's really just a simple typo in
your template.
Merge branch 'dont-nest-template-errors' of git://github.com/imlucas/gulp-jade
make sure jade error is instance of error