Handle user aborts during auth workflows in Phabricator

Summary: Depends on D6872. Ref T3687. Give the user a nice dialog instead of a bare exception.

Test Plan: Cancelled out of Twitter and JIRA workflows. We should probably do this for the OAuth2 workflows too, but they're a bit of a pain to de-auth and I am lazy.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3687

Differential Revision: https://secure.phabricator.com/D6873