Added useful warning for undecided condition

If step has a conition based on var but var has not been
specified in params, it is not possible to decide if this
condition is fulfilled.

This case cannot be copletely avoided, byu now it will
generate a useful warning.

Functional behavor is that such steps are *allowed* and
eventually revealed to tester.  (Possibly in a helpful manner.)

lib/SugarTrail/Template/Condition.pm

@@ -76,6 +76,15 @@ sub match {
     my $have    = $self->{v};       # what we have in this step
     my $want    = $params->{$name}; # what they want
 
+    unless ($want) {
+        warn sprintf (
+            "undecided condition: %s; did you forget to specify %s?\n",
+            $self->{string},
+            $name
+        );
+        return 1;
+    }
+
     # execute the right handler
     if (exists $self->{handlers}->{$op}) {
         return &{ $self->{handlers}->{$op} }($want, $have);