#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.

Would be great if you could be more specific - "Ui field" feels a little bit vague. 
Also, "errors" in this case also feels a tad unclear - is this a callstack - e.g. a multi line construct which will require a big QLabel spanning several lines in height or just a single line thing? 

I suggest changing the comment to:

```
:param error_field: QLabel object to use to display short error messages.
```