improve error messages in main somewhat

bin/main.ml

@@ -1,3 +1,5 @@
+include (val Logging.sublogs Server.logger "Main")
+
 let min_level =
   match Sys.getenv_opt "LOG_LEVEL" |> Option.map String.uppercase_ascii with
   | Some "TRACE" -> Logging.TRACE
@@ -72,4 +74,14 @@ let config : Server.config = {
 }
 
 let () =
-  Lwt_main.run @@ Server.run config
+  Printexc.register_printer
+    (function
+      | Unix.Unix_error (eno, who, _) -> Some (Fmt.str "%s: %s" who (Unix.error_message eno))
+      | Failure msg -> Some ("internal error: " ^ msg)
+      | Invalid_argument who -> Some ("internal error: invalid argumnet: " ^ who)
+      | _ -> None);
+  try
+    Lwt_main.run @@ Server.run config
+  with exn ->
+    error (fun m -> m "%a" Fmt.exn exn);
+    exit 1