Open editor from stack trace

Summary:
I've considered that user may have set editor but not checked out Phabricator repositories.
But stack trace is useful mainly for developers.

Test Plan:
Click on path in Unhandled Exception.
Repeat with disabled editor.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2107