[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/plugins/ssr/ssrview.ml b/plugins/ssr/ssrview.ml
index 88a3e85211..ad0a31622c 100644
--- a/plugins/ssr/ssrview.ml
+++ b/plugins/ssr/ssrview.ml
@@ -194,9 +194,11 @@ let interp_glob ist glob = Goal.enter_one ~__LOC__ begin fun goal ->
       Pp.(str"interp-out: " ++ Printer.pr_econstr_env env sigma term));
     tclUNIT (env,sigma,term)
   with e ->
+    (* XXX this is another catch all! *)
+    let e, info = Exninfo.capture e in
     Ssrprinters.ppdebug (lazy
     Pp.(str"interp-err: " ++ Printer.pr_glob_constr_env env glob));
-     tclZERO e
+    tclZERO ~info e
 end
 
 (* Commits the term to the monad *)