Require $key in msg() functions

Note: calling msg() with no parameter was never supported,
doing this on a RequestContext for example would result in:
PHP Warning:  Missing argument 1 for wfMessage() ...followed
by a bunch of fallout.

So this patch only formally declares what was already a
requirement in reality.

Change-Id: I1864afb8bcc641698689828914949a06506d8f3a