Provide more informative errors when oauth token not found

The common `oauth_listener needs an interactive environement` (e.g. #64) seems to often be due to `.httr-auth` not being where `drop_auth` expects it to be. This is a pretty unintuitive error message from `httr`; maybe we can wrap it a bit to let the user know we can't find the token?

Start by reproducing the `oauth_listener needs an interactive environement` error and inspect how `drop_auth` looks for `.httr-auth`. The work is done when a missing token produces an informative error explaining that the token could not be found instead of the unintuitive OAuth listener message.

Written by the indexing model from the issue text.