Make bin/search init messaging a little more consistent

Summary:
Ref T12450. This mostly just smooths out the text a little to improve consistency. Also:

• Use isWritable().
• Make the "skipping because not writable" message more clear and tailored.
• Try not to use the word "index" too much to avoid confusion with bin/search index -- instead, talk about "initialize a service".

Test Plan: Ran bin/search init with a couple of different (writable / not writable) configs, saw slightly clearer messaging.

Reviewers: chad, 20after4

Reviewed By: 20after4

Maniphest Tasks: T12450

Differential Revision: https://secure.phabricator.com/D17572