Add OverflowError and use it for small int overflow instead of assert.
diff --git a/py/objint.c b/py/objint.c
index 937bff7..59181ea 100644
--- a/py/objint.c
+++ b/py/objint.c
@@ -56,7 +56,7 @@
 
 // This is called only with strings whose value doesn't fit in SMALL_INT
 mp_obj_t mp_obj_new_int_from_long_str(const char *s) {
-    assert(0);
+    nlr_jump(mp_obj_new_exception_msg(MP_QSTR_OverflowError, "long int not supported in this build"));
     return mp_const_none;
 }
 
@@ -66,8 +66,7 @@
     if ((value & (WORD_MSBIT_HIGH | (WORD_MSBIT_HIGH >> 1))) == 0) {
         return MP_OBJ_NEW_SMALL_INT(value);
     }
-    // TODO: Raise exception
-    assert(0);
+    nlr_jump(mp_obj_new_exception_msg(MP_QSTR_OverflowError, "small int overflow"));
     return mp_const_none;
 }
 
@@ -75,8 +74,7 @@
     if (MP_OBJ_FITS_SMALL_INT(value)) {
         return MP_OBJ_NEW_SMALL_INT(value);
     }
-    // TODO: Raise exception
-    assert(0);
+    nlr_jump(mp_obj_new_exception_msg(MP_QSTR_OverflowError, "small int overflow"));
     return mp_const_none;
 }
 #endif