Add method to get the error code from the USB object.

[estranged42]

I needed to get the error code out of the USB object while debugging a project. There didn't seem to be a way to do that, so I added a really simple method to return it.

[dhalbert]

Do you want to try to submit this upstream to https://github.com/arduino/ArduinoCore-samd ?

[estranged42]

That probably makes more sense. :) It looks like I can't fork their repo while I have a fork of this one, so I think I'll have to delete my fork first which will probably break this PR, which is probably fine. Thanks for the suggestion!

[dhalbert]

Your fork is a fork of both our repo and theirs. You can check out one of their branches instead of ours. It's a bit painful to figure out (I had to do this too), but it's possible to remain with your current fork. But if that's too crazy-making, no problem, we can close this PR.

[estranged42]

Ah I should have guessed there was some way to do it. I've swapped out my fork and added the PR on the upstream repo: arduino#362

Thanks so much!