Give a good error message when waitpid raises

This behavior is best demonstrated by the report in #210, where perf
crashed and that made magic-trace crash. Let's nudge people that perf
may have crashed, instead.

src/trace.ml

@@ -356,7 +356,18 @@ module Make_commands (Backend : Backend_intf.S) = struct
         | Core_unix.Unix_error (_, (_ : string), (_ : string)) ->
           (* We raced, but it's OK because the child still exited. *)
           ());
-    let%bind.Deferred (_ : Core_unix.Exit_or_signal.t) = Async_unix.Unix.waitpid pid in
+    (* [Monitor.try_with] because [waitpid] raises if perf died before we got here. *)
+    let%bind.Deferred (waitpid_result : (Core_unix.Exit_or_signal.t, exn) result) =
+      Monitor.try_with (fun () -> Async_unix.Unix.waitpid pid)
+    in
+    (match waitpid_result with
+    | Ok _ -> ()
+    | Error error ->
+      Core.eprintf
+        !"Warning: [perf] exited suspiciously quickly; it may have crashed.\n\
+          Error: %{Exn}\n\
+          %!"
+        error);
     (* This is still a little racey, but it's the best we can do without pidfds. *)
     Ivar.fill exited_ivar ();
     let%bind () = detach attachment in