Detect incomplete installations and provide useful error

[jasonmp85]

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.

[jasonmp85]

To protect against users accidentally hitting a local table when pg_shard is not loaded, we could add a trigger to those tables (this could be part of master_create_distributed_table's logic) so that any queries involving them fail with a pg_shard related error message explaining that the table is distributed and they must LOAD pg_shard to use it.

[jasonmp85]

As for forgetting CREATE, this is a little tougher. One of us recently hit it while trying to run cstore_fdw's regression tests when pg_shard was also loaded. The COPY commands the tests were calling triggered pg_shard's logic. On one hand, we want the user to know about the misconfiguration, but on the other hand, putting anything in the output would break other test suites.

I'm thinking we send something only to the server log (possible?) to have a record that pg_shard is misconfigured, but otherwise just make the hook act as a no-op if the CREATE EXTENSION command hasn't yet been run. After all, this is probably a more rare case, since users must run CREATE EXTENSION to get access to the functions for distributing tables.