Fix generator error handling (Fix #89)

[regevbr]

Fix generator error handling (Fix #89)

When a generator's run emits an 'error' event, there is no handler for it, thus causing the application to crash.
In some cases the error is also passed to the callback of the run method, which can cause double handling of the error event.
The solution is to handle the event of the generator and call the callback with it, while ensuring the callback is only being called once.
If no callback was passed, keep backwards behavior and let the emitted error crash the application (as it is not handled). This is the expected behaviour when using the yo cli tool

[regevbr]

@SBoudrias can you please check?
I added required tests and made sure the tests fails before the fix (as mentioned in the first comment it can fail in 2 ways - un handled error event or callback being called twice - both were reproduced and fixed)

[SBoudrias]

Thanks!