OutputPage::showErrorPage raises a notice if arguments are incoherent

The method has the following signature:
OutputPage::showErrorPage( $title, $msg, $params = array() )

$msg can be a string or a Message object.

If it's a string, a Message object is built, with $params as parameters.
If it's a Message object, $params is ignored.

The core now triggers a notice in the case a call is made with $msg an instance
of Message object, and a (non-empty array) $params argument is given.

Change-Id: I227a416f088fc1acd6a04345ed0e24d06f967ecc
Merge "OutputPage::showErrorPage raises a notice if arguments are incoherent"