STM: 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 STM 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 AlwaysFail ()
```

lib/STM.ml

@@ -170,15 +170,15 @@ struct
     let rec interp_agree s sut cs = match cs with
       | [] -> true
       | c::cs ->
-        let res = Spec.run c sut in
+        let res = Util.tag_exn_with Spec.show_cmd (fun c -> Spec.run c sut) c in
         let b   = Spec.postcond c s res in
         let s'  = Spec.next_state c s in
         b && interp_agree s' sut cs
 
     let rec check_disagree s sut cs = match cs with
       | [] -> None
       | c::cs ->
-        let res = Spec.run c sut in
+        let res = Util.tag_exn_with Spec.show_cmd (fun c -> Spec.run c sut) c in
         let b   = Spec.postcond c s res in
         let s'  = Spec.next_state c s in
         if b