Added api to pass errors from generators

[KoryNunn]

No description provided.

[bguiz]

var y

[KoryNunn]

I have some reservations about the PR straight away, but I'm not sure if they are valid.

calling reject() does not necessarily halt execution at that point, but rather, will allow execution to continue to the next yield or return. This could cause side effects, just as it would if you allowed an errback style function to call callback(error) without returning.

I could solve this by throwing within the reject function, catching all throw errors, and detecting ones thrown by reject, swallowing, and passing the error, while re-throwing valid throws, but that would have a substantial impact on performance.

[bguiz]

I think it isn't worth the overhead. Also if you're using standard callbacks (without righto), you would already be accustomed to returning if the callback isn't at the last executable line of of the current function scope.

[bguiz]

perhaps change this to

if (x === 'x') { 
  return reject('foo');  
}

[bguiz]

So see the diff comment above, so the test now demonstrates that a return is expected, just like you would if a callback had occurred at that point.

Additional observation: Since reject takes the first argument as an error, perhaps we could just make it into a standard errback, so that 2nd arg is considered the result. So it could also be used to shortcut the iteration of the generator function. The return achieves this anyway, but would make for a more consistent experience with errbacks.

[KoryNunn]

I had coincided allowing to pass results there, but that does add ways to do the same task, which I'm not a huge fan of. It also wouldn't be a breaking change too add later, so, goat tracks and all that...

[bguiz]

but that does add ways to do the same task

'nuff said... merge!