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