Give "bin/config" a friendlier error message if "local.json" is not writable

Summary: Ref T13403. We currently emit a useful error message, but it's not tailored and has a stack trace. Since this is a relatively routine error and on the first-time-setup path, tailor it so it's a bit nicer.

Test Plan:

• Ran bin/config set ... with an unwritable "local.json".
• Ran bin/config set ... normally.

Maniphest Tasks: T13403

Differential Revision: https://secure.phabricator.com/D20779