Better logging of bundling errors

[brianmhunt]

I noticed that bundling errors seem to fall silent.
The simplest workaround is to add log.error('Bundle error:', error);.

I don't know Karma's internal workings well enough to speculate why done(error, null) does not report the error, but I noted the coffee plugin similarly logs a message with

log.error('%s\n  at %s:%d', e.message, file.originalPath, e.location.first_line);

Cheers! 🍻

[klaascuvelier]

Implemented by #5