Allow running with non-existent CONFIG_FILE

[PkmX]

Running harmonia without a settings.toml file present just gives an error:

[2024-08-20T20:00:42Z ERROR harmonia] Couldn't read config file 'settings.toml'
[2024-08-20T20:00:42Z ERROR harmonia] because: No such file or directory (os error 2)

I can workaround this by just making an empty settings.toml or run it with CONFIG_FILE=/dev/null, but it should be more user-friendly to just ignore the non-existent file and run with default settings.