fix: prevent panic on thread if no mount is selected

[HenryRoutson]

Just a simple fix
You can see the bug if you try to search before selecting a drive,
the drive is set to "" and an unwrap none panics a thread

something like

thread 'tokio-runtime-worker' panicked at 'called Option::unwrap() on a None value', src/search.rs:88:59
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
thread 'tokio-runtime-worker' panicked at 'called Result::unwrap() on an Err value: PoisonError { .. }', src/search.rs:85:34
thread 'notify-rs fsevents loop' panicked at 'called Result::unwrap() on an Err value: PoisonError { .. }', src/filesystem/cache.rs:48:48

I think this should be set to all mounts or the 0th by defualt, and outlined in the ui somehow

@conaticus

i think this may fix part of #26

[conaticus]

Thanks, I moved this to the client side and sent an alert if search was attempted outside of volume.
Also haven't forgot about #39, will test soon