Add more robust error handling to Document library.

[begedin]

Currently, depending on the error type, the Document library simply returns a message, without anything else. Due to that, apps using the add-on have to rely on searching for substrings within the error message in order to determine the type of error and act accordingly.

A good example of this behavior is https://github.com/venkatd/storypad/blob/development/app/controllers/error.js

I believe we should have a more robust solution in place. Something sending both a message and an error code of some sort, so the default message can be displayed if the app chooses to, but also so that more can be done in response to a message in a way that's more reliable than searching through strings.

I'm not sure what that better option is, though. I'll look into it.