Improve undefined region show message.

Fixes #1913

src/region.js

@@ -1,4 +1,4 @@
-/* jshint maxcomplexity: 10, maxstatements: 29 */
+/* jshint maxcomplexity: 10, maxstatements: 29, maxlen: 120 */
 
 // Region
 // ------
@@ -207,6 +207,13 @@ _.extend(Marionette.Region.prototype, Backbone.Events, {
   },
 
   _ensureViewIsIntact: function(view) {
+    if (!view) {
+      throw new Marionette.Error({
+        name: 'ViewNotValid',
+        message: 'The view passed is undefined and therefore invalid. You must pass a view instance to show.'
+      });
+    }
+
     if (view.isDestroyed) {
       throw new Marionette.Error({
         name: 'ViewDestroyedError',

test/unit/region.spec.js

@@ -858,4 +858,22 @@ describe('region', function() {
       expect(this.region.currentView).to.be.undefined;
     });
   });
+
+  describe('when showing undefined in a region', function() {
+    beforeEach(function() {
+      this.setFixtures('<div id="region"></div>');
+
+      this.region = new Backbone.Marionette.Region({
+        el: '#region'
+      });
+
+      this.insertUndefined = function() {
+        this.region.show(undefined);
+      }.bind(this);
+    });
+
+    it('should throw an error', function() {
+      expect(this.insertUndefined).to.throw('The view passed is undefined and therefore invalid. You must pass a view instance to show.');
+    });
+  });
 });