Add a some error context to indicate when pop-launcher is missing

[markstos]

Is your feature request related to a problem? Please describe.

Today I installed onagre for the first time and tried to start it but it immediately crashed:

onagre
Unable to load user theme: Unable to find theme settings file /home/mark/.config/onagre/theme.toml
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }', src/subscriptions/pop_launcher.rs:87:14
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Describe the solution you'd like

I expected the tool to work with a no configuration file. Either some defaults should be used, or it should see that my config file is missing and copy a default config file into place and then start.

[oknozor]

Hello @markstos onagre does launch without a config file. The thread panic occurs because you don't have pop-launcher installed on your system I think.

[markstos]

Thanks. Perhaps it could fail more gracefully if pop launcher does not exist.