#26089 Added "not found" error dialog when running in the deskop engine.

When apps are launched from the desktop engine, and the configured path for an app isn't working, the following message is being emitted:

![screen shot 2014-08-06 at 00 52 35](https://cloud.githubusercontent.com/assets/337710/3820650/db51e7b0-1cfb-11e4-8b64-117fc9e876db.png)

Clicking the button will launch a web browser and the close the dialog.

 
I thought about that, and i was wasn't sure what the right thing to do was. With the shotgun engine, that means that a different looking message box would pop up instead of the standard built-in javascript popup. Potentially with issues with parenting, worst case opening up behind shotgun! For the shell engine, since it is shell based, it doesn't feel bad if the feedback is delivered via the console. But the shotgun engine and window parenting was probably my biggest concern.

One other reason to do it this way is that we can release the app any time, even before we have finished the target docs, because no-one will be affected by the change until we release the desktop. So that seemed to reduce a bit of release complexity... :)