Add error callback for showStep (Issue #274) (#291)

* Add error callback for showStep

Calling showStep without a proper step target fails, but the tour error
callback is not invoked. This change fixes that, and sets the current
step number to the step that failed for debugging purposes

* Revert step num after error callback

Not sure if this will work

* remove all stepNum stuff, see if it still breaks

just a test to see what's breaking

* Add templates stuff

* Revert "Add templates stuff"

This reverts commit 8dfed46.

* Resubmit error callback patch

temporarily set currStepNum then revert

* deglobalizing temporary var

* Hoist temp var to top of function

src/js/hopscotch.js

@@ -1943,8 +1943,12 @@
      * @returns {Object} Hopscotch
      */
     this.showStep = function(stepNum) {
-      var step = currTour.steps[stepNum];
+      var step = currTour.steps[stepNum],
+          prevStepNum = currStepNum;
       if(!utils.getStepTarget(step)) {
+        currStepNum = stepNum;
+        utils.invokeEventCallbacks('error');
+        currStepNum = prevStepNum;
         return;
       }