R1:e7c76d92d546

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…