Fix error message when deleting key providers

Error messages are not supposed to be proper sentences, and according to
the error style guide "cannot" is preferable over "can't".

Also add an appropriate errcode.

contrib/pg_tde/expected/default_principal_key.out

@@ -45,7 +45,7 @@ SELECT provider_id, provider_name, key_name
 
 -- fails
 SELECT pg_tde_delete_global_key_provider('file-provider');
-ERROR:  Can't delete a provider which is currently in use
+ERROR:  cannot delete provider which is currently in use
 SELECT id, name FROM pg_tde_list_all_global_key_providers();
  id |     name      
 ----+---------------

contrib/pg_tde/expected/key_provider.out

@@ -91,7 +91,7 @@ SELECT id, name FROM pg_tde_list_all_global_key_providers();
 
 -- fails
 SELECT pg_tde_delete_database_key_provider('file-provider');
-ERROR:  Can't delete a provider which is currently in use
+ERROR:  cannot delete provider which is currently in use
 SELECT id, name FROM pg_tde_list_all_database_key_providers();
  id |      name      
 ----+----------------
@@ -127,7 +127,7 @@ SELECT pg_tde_set_key_using_global_key_provider('test-db-key', 'file-keyring');
 
 -- fails
 SELECT pg_tde_delete_global_key_provider('file-keyring');
-ERROR:  Can't delete a provider which is currently in use
+ERROR:  cannot delete provider which is currently in use
 SELECT id, name FROM pg_tde_list_all_global_key_providers();
  id |     name      
 ----+---------------

contrib/pg_tde/src/catalog/tde_keyring.c

@@ -309,7 +309,8 @@ pg_tde_delete_key_provider_internal(PG_FUNCTION_ARGS, Oid db_oid)
 	if (provider_used)
 	{
 		ereport(ERROR,
-				errmsg("Can't delete a provider which is currently in use"));
+				errcode(ERRCODE_OBJECT_IN_USE),
+				errmsg("cannot delete provider which is currently in use"));
 	}
 
 	delete_key_provider_info(provider_name, db_oid);