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.