parserMultiAnswer, check if user checker is a CODE ref.

Check if any user defined checker is a subroutine, and give an error
message if not, instead of sliently overwriting it with a default checker.

macros/parsers/parserMultiAnswer.pl

@@ -102,6 +102,7 @@ sub cmp {
 		}
 	}
 
+	die "Your checker must be a subroutine." if defined($self->{checker}) && ref($self->{checker}) ne 'CODE';
 	unless (ref($self->{checker}) eq 'CODE') {
 		$self->{checker} = sub {
 			my ($correct, $student, $self, $ans) = @_;