vm: Make sure that exception triple is <type, instance, traceback>.

This reduntant triple is one of the ugliest parts of Python, which they
chickened out to fix in Python3. We really should consider passing just
as single exception instance (without breaking Python-level APIs of course),
but until we do, let's follow CPython layout.

py/vm.c

@@ -415,8 +415,8 @@ unwind_jump:
                         // if TOS is None, just pops it and continues
                         // if TOS is an integer, does something else
                         // else error
-                        if (mp_obj_is_exception_instance(TOP())) {
-                            nlr_jump(TOP());
+                        if (mp_obj_is_exception_type(TOP())) {
+                            nlr_jump(sp[-1]);
                         }
                         if (TOP() == mp_const_none) {
                             sp--;
@@ -716,7 +716,7 @@ unwind_return:
                 // push(traceback, exc-val, exc-type)
                 PUSH(mp_const_none);
                 PUSH(nlr.ret_val);
-                PUSH(nlr.ret_val); // TODO should be type(nlr.ret_val), I think...
+                PUSH(mp_obj_get_type(nlr.ret_val));
 
             } else {
                 // propagate exception to higher level