Changed error message for closed connection

Since SQLALchemy test suite checks for the word
'Invalid' we changed the error message for closed
connections back to "Invalid connection or not connected"

mariadb/connections.py

@@ -41,7 +41,7 @@ class Connection(mariadb._mariadb.connection):
 
     def _check_closed(self):
         if self._closed:
-            raise mariadb.ProgrammingError("Connection is closed")
+            raise mariadb.ProgrammingError("Invalid connection or not connected")
 
     def __init__(self, *args, **kwargs):
         """