Fix Beanstalk exception handling

This was discovered as part of the Python 3 work for Beanstalk in #2372. Specifically, see:

https://github.com/boto/boto/pull/2372#issuecomment-48577170

The workaround posted there is what's in this commit, except that is also rstrips `Exception` from the error code name if present to prevent having to redefine all the exception class names. I've gone through the [docs](http://docs.aws.amazon.com/elasticbeanstalk/latest/api/Welcome.html) (every page... ugh) and confirmed that the exception list is up to date.

Integration tests now pass, which they apparently haven't been for a while. Once this is merged I can merge in the Python 3 support.

@jamesls, @felixonmars please take a look.