py: Add MP_OBJ_STOP_ITERATION and make good use of it.

Also make consistent use of MP_OBJ_NOT_SUPPORTED and MP_OBJ_NULL.
This helps a lot in debugging and understanding of function API.
diff --git a/py/objfloat.c b/py/objfloat.c
index 16d4fbf..8041019 100644
--- a/py/objfloat.c
+++ b/py/objfloat.c
@@ -140,7 +140,7 @@
         case MP_BINARY_OP_MORE_EQUAL: return MP_BOOL(lhs_val >= rhs_val);
 
         default:
-            return MP_OBJ_NULL; // op not supported
+            return MP_OBJ_NOT_SUPPORTED;
     }
     return mp_obj_new_float(lhs_val);
 }