R1:d69a6b8490a5

Improve error messages when specifying bad set or list to bin/config

Summary: Fixes T7308. Multiple users have encountered confusion around how they should specify a set or list in JSON; provide examples.

Test Plan:
```
epriestley@orbital ~/dev/phabricator $ ./bin/config set files.image-mime-types true
Usage Exception: Config key 'files.image-mime-types' is of type 'set'. Specify it in JSON. For example:

./bin/config set '{"value1": true, "value2": true}'

epriestley@orbital ~/dev/phabricator $ .