exceptions::too_big sounds like std::range_error to me.

With good reason -- it's essentially the same thing. But in Josuttis' "The C++ Standard Library", the wording on std::range_error is that it's "used to report a range error in internal computations." That didn't quite sound like what I was trying to convey with that exception, which is that the integer_t that the user is trying to convert won't fit into the type that he's trying to convert it to. It does inherit from std::range_error though, so you can use either for most things, and get more specific if desired.

In addition, it seemed discordant to me to have a library that almost always throws exceptions from a particular namespace. It was almost no additional work to make it always throw from a specific namespace, and it fit my sense of rightness.