Improve the “unable to find key” warning

Improves the missing key warning by logging of the property that can read for example:

var context = { foo: true };

var scope = new Scope(context);
var scopeKeyData = scope.computeData("foo.length", {
   warnOnMissingKey: true
});

scopeKeyData.read();  // -> Unable to find key "foo.length". Found "foo" with value: true