Updated error handling for Dropbox example
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)

huxuan/flask-oauthlib,adambard/flask-oauthlib,icook/flask-oauthlib,lepture/flask-oauthlib,icook/flask-oauthlib,brightforme/flask-oauthlib,PyBossa/flask-oauthlib,landler/flask-oauthlib,landler/flask-oauthlib,Fleurer/flask-oauthlib,tonyseek/flask-oauthlib,brightforme/flask-oauthlib,PyBossa/flask-oauthlib,CoreyHyllested/flask-oauthlib,Ryan-K/flask-oauthlib,cogniteev/flask-oauthlib,auerj/flask-oauthlib,adambard/flask-oauthlib,cogniteev/flask-oauthlib,CommonsCloud/CommonsCloud-FlaskOAuthlib,stianpr/flask-oauthlib,tonyseek/flask-oauthlib,stianpr/flask-oauthlib,Fleurer/flask-oauthlib,auerj/flask-oauthlib,Ryan-K/flask-oauthlib,CommonsCloud/CommonsCloud-FlaskOAuthlib,huxuan/flask-oauthlib,lepture/flask-oauthlib,CoreyHyllested/flask-oauthlib