Lin: Print commands that trigger uncaught exceptions

The standard QCheck mechanism to catch exceptions that are raised in
tests do not give much clue about the source (ie Lin command) of that
exception
This packs the unsoundly-specified command with the exception in such a
case so that it can be displayed

Before:
```
exception Failure("unexpected")
```

After:
```
exception Failure("unexpected") raised but not caught while running always_fail t
```

Author: shym

lib/lin.ml

@@ -479,6 +479,9 @@ module MakeCmd (ApiSpec : Spec) : Internal.CmdSpec = struct
       apply_f (f arg) rem state
 
   let run cmd state =
-    let Cmd { args ; rty ; f ; _ } = cmd in
-    Res (rty, apply_f f args state)
+    Util.tag_exn_with show_cmd
+      (fun cmd ->
+        let (Cmd { args; rty; f; _ }) = cmd in
+        Res (rty, apply_f f args state))
+      cmd
 end