gracefully handle babelify errors in gulp (fixes #247)

[cvan]

when you cause a babelify error, the 'babelify' gulp task would never finish, causing the 'watch' task to never finish. so you'd have to restart gulp every time that happened.

while I was at it, I fixed the error message and formatting so we actually return the entire stack trace instead of just the error message. :tada:

before

after

[cvan]

we could do some long-term solution like using https://www.timroes.de/2015/01/06/proper-error-handling-in-gulp-js/

but I feel like this is sufficient. thoughts?

[KevinGrandon]

This seems fine to me for now, thanks! R+!

[cvan]

thanks :)