Search: display error messaging for unsupported custom taxonomies

The instant search index uses a list of custom taxonomies to decide what to include in the index. Right now if the end user configures a custom taxonomy that is not in the allowed list then it just doesn't show up and there is no messaging to lead the user down the right path.

Roughly we have:
- hard coded on wp.com for the indexing in class.jetpack-sync-module-search.php
- that code is intended to by synced to jetpack but isn't yet
- We should then check against it in some way for when a user tries to configure a filter that is not yet in the index
- have some path for the end user to open a PR to request a new field get added, or encourage them to use a taxonomy that we already support

Start with the hard-coded taxonomy list in class.jetpack-sync-module-search.php and trace how custom taxonomy filters are configured and synced between wp.com and Jetpack. Define how unsupported taxonomies should be detected, what guidance or request path users should see, and how supported-taxonomy behavior remains unchanged.

Written by the indexing model from the issue text.