#26321: [Desktop] Unable to click on the back button to change config path

*Story*
As a Toolkit user setting up a project, I want to be able to click on the back button to change the configuration folder path is I enter an invalid value.

*Description*
When setting up a project, if you set the config and the storage location with the same path you will get an error saying that you can't do that. (See the attached file)
When getting this error, the only option you have is to cancel the set up.

The general idea would be to add a pre-setup validation method to the wizard and then update the GUI so that right before it goes to the setup page it calls it and if there is a failure it should go to a page that reports on the error and makes you go back and fix it rather than the setup.

You are squashing an exception down to a single line. This is often useless, you may get something like `KeyError: 'path'` - you often really need the full call stack for this to make sense. 

Our standard approach is that for `TankError`s, we present the message only, and for general exceptions, we show the entire callstack. So my suggestion would be to catch `TankError`s only rather than all `Exceptions`.