Provide better message when CREATE EXTENSION can't find a target schema.

The new message (and SQLSTATE) matches the corresponding error cases in
namespace.c.

This was thought to be a "can't happen" case when extension.c was written,
so we didn't think hard about how to report it.  But it definitely can
happen in 9.2 and later, since we no longer require search_path to contain
any valid schema names.  It's probably also possible in 9.1 if search_path
came from a noninteractive source.  So, back-patch to all releases
containing this code.

Per report from Sean Chittenden, though this isn't exactly his patch.

src/backend/commands/extension.c

@@ -1391,12 +1391,16 @@ CreateExtension(CreateExtensionStmt *stmt)
 		 */
 		List	   *search_path = fetch_search_path(false);
 
-		if (search_path == NIL) /* probably can't happen */
-			elog(ERROR, "there is no default creation target");
+		if (search_path == NIL)	/* nothing valid in search_path? */
+			ereport(ERROR,
+					(errcode(ERRCODE_UNDEFINED_SCHEMA),
+					 errmsg("no schema has been selected to create in")));
 		schemaOid = linitial_oid(search_path);
 		schemaName = get_namespace_name(schemaOid);
 		if (schemaName == NULL) /* recently-deleted namespace? */
-			elog(ERROR, "there is no default creation target");
+			ereport(ERROR,
+					(errcode(ERRCODE_UNDEFINED_SCHEMA),
+					 errmsg("no schema has been selected to create in")));
 
 		list_free(search_path);
 	}