Check that provider has set wal_level=logical and otherwise immediately fail in pglogical.create_subscription.

[PaulJuliusMartinez]

Currently, if attempting to create a subscription to a node that has not set wal_level=logical, the call to pglogical.create_subscription will succeed, but the node will appear in a down state and users will have to check the logs to see that the replication slot was unable to be created because the provider has wal_level < logical.

Since we already synchronously connect to the provider when creating a subscription, there's no reason to not verify that they have the correct wal_level setting, and immediately tell the user what is wrong.