Resolves #38 (filer.open actually ignores onError)

Hi there, Eric!

I just believe that filer.open should not ignore second param, onError (just as it claimed in docs).
Here's a small fix for this.

If this was intentional or this pull request is inappropriate for some other reason, please, just let all of us know.
If for some reasons this PR should be somehow improve, I'll be glad to fix all that needed.

It's just that, from my practise, it is almost always better to create issue and a pull request than only issue.
Especially when it looks like you are talking about bug, not some new functionality :)