[exn] [tactics] improve backtraces on monadic errors

Current backtraces for tactics leave a bit to desire, for example
given the program:

```coq
Lemma u n : n + 0 = n.
rewrite plus_O_n.
```

the backtrace stops at:

```
Found no subterm matching "0 + ?M160" in the current goal.
Called from file "proofs/proof.ml", line 381, characters 4-42
Called from file "tactics/pfedit.ml", line 102, characters 31-58
Called from file "plugins/ltac/g_ltac.mlg", line 378, characters 8-84
```

Backtrace information `?info` is as of today optional in some tactics,
such as `tclZERO`, it doesn't cost a lot  however to reify backtrace
information indeed in `tclZERO` and provide backtraces for all tactic
errors. The cost should be small if we are not in debug mode.

The backtrace for the failed rewrite is now:

```
Found no subterm matching "0 + ?M160" in the current goal.
Raised at file "pretyping/unification.ml", line 1827, characters 14-73
Called from file "pretyping/unification.ml", line 1929, characters 17-53
Called from file "pretyping/unification.ml", line 1948, characters 22-72
Called from file "pretyping/unification.ml", line 2020, characters 14-56
Re-raised at file "pretyping/unification.ml", line 2021, characters 66-73
Called from file "proofs/clenv.ml", line 254, characters 12-58
Called from file "proofs/clenvtac.ml", line 95, characters 16-53
Called from file "engine/proofview.ml", line 1110, characters 40-46
Called from file "engine/proofview.ml", line 1115, characters 10-34
Re-raised at file "clib/exninfo.ml", line 82, characters 4-38
Called from file "proofs/proof.ml", line 381, characters 4-42
Called from file "tactics/pfedit.ml", line 102, characters 31-58
Called from file "plugins/ltac/g_ltac.mlg", line 378, characters 8-84
```

which IMO is much better.

1 files changed, 3 insertions, 1 deletions

diff --git a/tactics/hints.ml b/tactics/hints.ml
index 5fb519cc4f..a0670ef70d 100644
--- a/tactics/hints.ml
+++ b/tactics/hints.ml
@@ -1570,6 +1570,8 @@ let run_hint tac k = match warn_hint () with
   else Proofview.tclTHEN (log_hint tac) (k tac.obj)
 | HintStrict ->
   if is_imported tac then k tac.obj
-  else Proofview.tclZERO (UserError (None, (str "Tactic failure.")))
+  else
+    let info = Exninfo.reify () in
+    Proofview.tclZERO ~info (UserError (None, (str "Tactic failure.")))
 
 let repr_hint h = h.obj