bigint.c (mrb_bint_as_int): change overflow message.

From "integer too big" to "integer out of range" because the integer can
be too small if it's negative.

mrbgems/mruby-bigint/core/bigint.c

@@ -1111,7 +1111,7 @@ mrb_bint_as_int(mrb_state *mrb, mrb_value x)
   mrb_int i;
 
   if (!mpz_get_int(&b->mp, &i)) {
-    mrb_raise(mrb, E_RANGE_ERROR, "integer too big");
+    mrb_raise(mrb, E_RANGE_ERROR, "integer out of range");
   }
   return i;
 }