Lin: Print commands that trigger unexpected exceptions

The standard QCheck mechanism to catch exceptions that are raised in
tests do not give much clues about the source (ie Lin command) of that
exception
So this creates a specific Unexpected_exception that will pack the
command along with the original exception so that it can be explained
when the tests errors out

Before:
```
exception Invalid_argument("Bytes.blit")
```

After:
```
exception Invalid_argument("Bytes.blit") unexpected in command Buffer.add_string t ""
```

lib/lin.ml

@@ -1,3 +1,14 @@
+exception Unexpected_exception of string * exn
+
+let print_unhandled_exception e =
+  match e with
+  | Unexpected_exception (cmd, exc) ->
+      Some (Format.sprintf "%s unexpected in command %s" (Printexc.to_string exc) cmd)
+  | _ -> None
+
+let _ =
+  Printexc.register_printer print_unhandled_exception
+
 module Internal =
 struct
   open QCheck
@@ -464,5 +475,6 @@ module MakeCmd (ApiSpec : Spec) : Internal.CmdSpec = struct
 
   let run cmd state =
     let Cmd { args ; rty ; f ; _ } = cmd in
-    Res (rty, apply_f f args state)
+    try Res (rty, apply_f f args state)
+    with e -> raise (Unexpected_exception (show_cmd cmd, e))
 end