Detect incomplete installations and provide useful error

Not entirely sure whether this is a feature request or a bug report, but various emails have been hitting my inbox of users either:
- Putting `pg_shard` in `shared_preload_libraries` but forgetting to call `CREATE EXTENSION`
- Calling `CREATE EXTENSION` but forgetting to ever `LOAD` `pg_shard` (using the above config variable or otherwise)

These causes produce a variety of symptoms, but common ones are:
- Running a query against a distributed table but actually hitting the local table because `pg_shard`'s hooks have not been loaded (caused by lack of `LOAD`)
- Calling a utility command and having it fail with the error `schema "pgs_distribution_metadata" does not exist` (caused by lack of `CREATE`)

This is biting enough people that some runtime guidance could help out a lot.