Config - improve lock message for option `phabricator.uninstalled-applications`

Summary: Fixes T6175. This particular option is only editable via the Applications application so let the user know that.

Test Plan: saw error message and clicked through to applications app. tried another locked option and saw old message

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6175

Differential Revision: https://secure.phabricator.com/D10841