Skip the final 'command opam xxx failed' unless in verbose mode

this is redundant since there must be more precise error messages, and we already
got the return value of the program for that

src/client/opamArg.ml

@@ -1392,7 +1392,8 @@ let run default commands =
   with
   | OpamGlobals.Exit 0 -> ()
   | e                  ->
-    Printf.eprintf "'%s' failed.\n" (String.concat " " (Array.to_list Sys.argv));
+    if !OpamGlobals.verbose then
+      Printf.eprintf "'%s' failed.\n" (String.concat " " (Array.to_list Sys.argv));
     let exit_code = ref 1 in
     begin match e with
       | OpamGlobals.Exit i ->