Handle errors in bluetooth stack

## Prerequisites
- [X] I have searched open and closed issues for duplicates.
I believe this is different from #80, but if it is a duplicate you can close it.

## Feature
When something fails in the bluetooth stack (like bluez errors), the GUI doesn't register the failure and acts as if it is performing a Bluetooth device search as usual. As it is, it's impossible to tell if no devices are available or if bluetooth is failing for some reason.

Current behavior: if I toggle the bluetooth status switch to "on", the settings page lights up and the "Searching..." spinner appears.

To recreate this behavior, you can replicate the state of my system when I discovered it. There may be other issues that could cause Bluetooth to fail but keep the Switchboard plug behaving normally, but I am unaware of them.

- First, turn Bluetooth off using usual methods (status bar switch, etc).
- Using rfkill, lock Bluetooth. In my case, Bluetooth had identity 0, so the command was `rfkill block 0`.
- Then, go to the Bluetooth settings page and toggle it back on. The page will behave exactly as if Bluetooth was functioning, but no new devices will appear and any existing paired devices will be unavailable to connect. This is the behavior I wish to see improved.
- To return your system to its regular state, execute `rfkill unblock $BLUETOOTH_ID` to remove the lock.

In my case, rfkill had been set to block bluetooth and I didn't know. Bluez would error because of this and print messages to the logs, but the plug would behave status as if it were functioning normally. I was unaware that there was a configuration issue at all until I looked through my system log.

**Describe the solution you'd like**

It would be helpful to include a failure message of some sort. Even if it doesn't include specific information about the error, indicating a failure would inform the user of an issue and prompt them to look for solutions.

(Edited to more clearly describe the issue and provide steps to replicate the behavior)