Add PhutilAuthUserAbortedException to libphutil
Summary:
Ref T3687. When users hit "Deny" or "Cancel" on web-based auth workflows, provide a way for adapters to indicate this.
With the next diff, this improves the UX of cancelling (a bare exception becomes a dialog).
Test Plan: Cancelled out of JIRA oauth workflow, got nice dialog (after next change).
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T3687
Differential Revision: https://secure.phabricator.com/D6872