Show the urlbar Connect button during a bootstrap or final error

At the moment the urlbar "Connect" button is hidden when:

1. We are in the middle of a bootstrap.
2. We are in the final error stage.

I think it might make sense to instead keep showing this button in its grey (non-purple) form where it just opens about:torconnect as a way for users to return to this page if they have closed the about:torconnect tab. Which is how the "Connect" button at the top of about:preferences#connection currently works. Otherwise, the only way back is to:

1. Try and visit a website, which redirects to about:torconnect.
2. Enter the about:torconnect url manually.
3. Edit your bridges and press the "Connect" button.
4. Click the "Connect" button at the top of about:preferences#connection.

/cc @donuts @felicia