Small documentation fix
Textual improvement

Using `in favor of` instead of `in favor for`; see http://en.wiktionary.org/wiki/in_favor_of
Updated error handling for Dropbox example

According to https://www.dropbox.com/developers/core/docs#oa2-authorize there is no `error_reason` but rather an `error` code (per Section 4.1.2.1 of the OAuth 2.0 spec)