py: Change nlr_jump to nlr_raise, to aid in debugging.

This does not affect code size or performance when debugging turned off.

To address issue #420.
diff --git a/py/objdict.c b/py/objdict.c
index f6070a0..c09ccfb 100644
--- a/py/objdict.c
+++ b/py/objdict.c
@@ -43,7 +43,7 @@
             assert(false);
 
         default:
-            nlr_jump(mp_obj_new_exception_msg_varg(&mp_type_TypeError, "dict takes at most 1 argument"));
+            nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError, "dict takes at most 1 argument"));
     }
 
     // add to the new dict any keyword args
@@ -71,7 +71,7 @@
             // dict load
             mp_map_elem_t *elem = mp_map_lookup(&o->map, rhs_in, MP_MAP_LOOKUP);
             if (elem == NULL) {
-                nlr_jump(mp_obj_new_exception_msg(&mp_type_KeyError, "<value>"));
+                nlr_raise(mp_obj_new_exception_msg(&mp_type_KeyError, "<value>"));
             } else {
                 return elem->value;
             }
@@ -199,7 +199,7 @@
     if (elem == NULL || elem->value == NULL) {
         if (deflt == NULL) {
             if (lookup_kind == MP_MAP_LOOKUP_REMOVE_IF_FOUND) {
-                nlr_jump(mp_obj_new_exception_msg(&mp_type_KeyError, "<value>"));
+                nlr_raise(mp_obj_new_exception_msg(&mp_type_KeyError, "<value>"));
             } else {
                 value = mp_const_none;
             }
@@ -258,7 +258,7 @@
     assert(MP_OBJ_IS_TYPE(self_in, &mp_type_dict));
     mp_obj_dict_t *self = self_in;
     if (self->map.used == 0) {
-        nlr_jump(mp_obj_new_exception_msg(&mp_type_KeyError, "popitem(): dictionary is empty"));
+        nlr_raise(mp_obj_new_exception_msg(&mp_type_KeyError, "popitem(): dictionary is empty"));
     }
     mp_obj_dict_it_t *iter = mp_obj_new_dict_iterator(self, 0);
 
@@ -287,7 +287,7 @@
         if (key == MP_OBJ_NULL
             || value == MP_OBJ_NULL
             || stop != MP_OBJ_NULL) {
-            nlr_jump(mp_obj_new_exception_msg(
+            nlr_raise(mp_obj_new_exception_msg(
                          &mp_type_ValueError,
                          "dictionary update sequence has the wrong length"));
         } else {