Log when Message::__toString has an unexpected format

Log when Message::__toString has an unexpected format

Message formatting methods have a side effect on how string conversion
will work, which is a security problem waiting to happen:

    $msg = new Message( 'foo' );
    echo $msg; // parsed
    echo $msg->plain();
    echo $msg; // not parsed

This change logs Message -> string transformations which are
affected by a prior call in this way. The behavior will be removed
in a later patch (possibly replaced by something more explicit
if it turns out that something depends on it).

Bug: T146416
Change-Id: Id51cf6a5a937bc41a914f317e980ef42e4d385fb