settings-widget: show failure messages when connecting or pairing fails

Adds a popup gtk_message_dialog when the bluetooth controller is unable to pair to a device. When connecting fails, show an error message via a toast in the properties window or preferences page. This provides feedback that something went wrong instead of silently failing.

Fixes #18. Also discussed in gnome-settings#1054. Previous work in !37 (closed). Based partly off of Sam Hewitt's design work here.