[ticket/11348] Wrong error message for pending account activation.

Misleading error message is shown when a user tries to log in with
a user acoount which is yet to be approved by an admin. It states that
user should contact the admin for activation. Changed lang variable
'ACTIVE_ERROR' to hold the correct error message.

PHPBB3-11348
[ticket/11348] Adding seperate user activation errors

Added separated error messages for inactive users in both admin
and email activation methods. Added two language messages into
common.php

PHPBB3-11348