Crashes when an error occurs inside the callback & no error handlers are attached

[phated]

Ref gulpjs/gulp#2111 and the comments in #34

It seems that this is a bug and it isn't likely that someone would rely on the "crashing" behavior (plus, it was never spec'ed by the tests). However, someone might want to attach an error listener and do something with the errors (outside of the gulp task system), so we should keep emitting errors. We can guard against the crash behavior by looking up if there's any handlers for 'error' and only emitting if any exist.