use std::exception as base class to z3_exception

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/math/subpaving/tactic/subpaving_tactic.cpp

@@ -252,7 +252,7 @@ public:
         }
         catch (z3_exception & ex) {
             // convert all Z3 exceptions into tactic exceptions
-            throw tactic_exception(ex.msg());
+            throw tactic_exception(ex.what());
         }
     }