Do not quote failure errors.

solver.ml

@@ -302,7 +302,7 @@ let build_command ~raw_options confg_solver property model =
       Stdlib.close_out out_channel;
       Ok tmp
     with Sys_error e ->
-      Error (Format.sprintf "Unexpected failure@ `%s'." e)
+      Error (Format.sprintf "Unexpected failure:@ %s." e)
   end >>= fun prop ->
   (* Build actual command-line. *)
   try
@@ -335,7 +335,7 @@ let build_command ~raw_options confg_solver property model =
     in
     Ok (prop, cmd)
   with Failure e ->
-    Error (Format.sprintf "Unexpected failure@ `%s'." e)
+    Error (Format.sprintf "Unexpected failure:@ %s." e)
 
 let exec ~raw_options config model property =
   let open Result in