Message: Improve validation for fallback keys

It's an easy mistake to write:

$msgSpec = [ $key, $param1, $param2 ];
wfMessage( $msgSpec );                    // WRONG

instead of

wfMessage( ...$msgSpec );                 // correct
Message::newFromSpecifier( $msgSpec );    // correct

If you use the wrong form, your intended message parameters are used
as fallback message keys, and usually ignored (assuming your intended
message key exists).

It's not possible to catch this mistake in all cases, since it's all
arrays of strings most of the time. But we can try.

Change-Id: I0b40ad90a352dc640e64090cac4224219752dc01