Refactor error types and add config file parsing

1. Main binary now parses a config toml and uses its paths for the
   output pcap and qmdl files
2. Previously we were relying on DiagDeviceError for several things
   that aren't a diag device. This modularizes that error in a way
   that both improves the error descriptions, and also allows for
   better separation of concerns.

Cargo.toml

@@ -27,4 +27,7 @@ libc = "0.2.150"
 log = "0.4.20"
 pcap-file = "2.0.0"
 thiserror = "1.0.50"
+# TODO: split into binary-only dependencies
+toml = "0.8.8"
+serde = { version = "1.0.193", features = ["derive"] }