Return result of Command::execute directly from main

Since `Result` implements
`Termination` (https://doc.rust-lang.org/std/process/trait.Termination.html),
the result of `Command::execute` can be returned from `main` directly.
If an `FF2MpvError` error would be propagated to `main`, it will be
printed using `eprintln` just as before, and the program will terminate
with an appropriate exit code:
https://doc.rust-lang.org/std/process/struct.ExitCode.html#associatedconstant.FAILURE

The only caveat is that `FF2MpvError` has to implement `Debug`, but
instead of deriving it I cheated a bit and re-used the existing
`Display` implementation to preserve the format of the error messages.

src/error.rs

@@ -1,6 +1,6 @@
 use std::io;
 use std::fmt;
-use std::fmt::Display;
+use std::fmt::Debug;
 
 pub enum FF2MpvError {
     NoConfig,
@@ -20,7 +20,7 @@ impl From<serde_json::Error> for FF2MpvError {
     }
 }
 
-impl Display for FF2MpvError {
+impl Debug for FF2MpvError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             Self::NoConfig => write!(f, "Config doesn't exist"),

src/main.rs

@@ -1,18 +1,14 @@
 use std::env;
-use std::process;
 
-use ff2mpv_rust::command::Command;
+use ff2mpv_rust::{command::Command, error::FF2MpvError};
 
-fn main() {
+fn main() -> Result<(), FF2MpvError> {
     let mut args = env::args();
     args.next(); // Skip binary path
 
     let command_name = args.next().unwrap_or_default();
     let command = get_command(&command_name);
-    if let Err(e) = command.execute() {
-        eprintln!("{e}");
-        process::exit(-1);
-    }
+    command.execute()
 }
 
 fn get_command(name: &str) -> Command {