Don't clobber app error handlers for non-API endpoints

I'd like to suggest this changeset as a fix for [this](https://github.com/twilio/flask-restful/issues/8) issue, and I think it's generally a better idea than replacing the app's error handler for all endpoints.

This is somewhat related to [this](https://github.com/twilio/flask-restful/pull/9) pull request, which didn't really fix the bad beahviour, but at least made it optional.

I believe this changeset *can* be merged as-is, I'm using it in my codebase and it works well for my needs. However, this is a tricky subject and Flask doesn't provide the exact APIs necessary to really hit the nail on the head, so I'm also proposing it as a basis to discussion. I'd love to hear what you think.