1070585: GUI no longer locks on connection test. Adds cancel button.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1070585

Exit early instead of nesting everything under the try.

```
def _clear_progress_bar(self):
    if not self.progress_bar:
        return

   self.progress_bar.hide()
   gobject.source_remove(self.timer)
   self.timer = 0
   self.progress_bar = None
```