assistant: Do not close vm assistant window on error cases

We need the VM assistant window to stay open so 'show_error()' can display the error message, when 'Create' button is clicked.

Steps to reproduce:

Remove write permissions on storage pool dir (as below) and create VM:

$ chmod -w ~/.local/share/gnome-boxes/images/

Error message will now be displayed with this MR (as below), which was not the case previously.