#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.

 
Any reason this is desktop specific?  Should it just test is engine has_ui?
That way for shell/shotgun it would show up if PySide/PyQt is installed.