[lmi] Use of range_error standard derivatives

[Greg Chicares]

The standard gives virtually no guidance for the use of these classes:
  range_error: "range errors in internal computations"
  overflow_error: "arithmetic overflow error"
  underflow_error: "arithmetic underflow error"
The only apparent reference to any of them in TC++PL4 actually refers
to ERANGE. I tend to conclude that any reasonable use is good.

The reason why I raise this matter now is to ask which exceptions
bourn_cast should throw. Right now, it looks like this:

    if(! to_traits::is_signed && from < 0)
        throw std::runtime_error("Cast would convert negative to unsigned.");
    if(from_traits::is_signed && from < to_traits::lowest())
        throw std::runtime_error("Cast would transgress lower limit.");
    if(to_traits::max() < from)
        throw std::runtime_error("Cast would transgress upper limit.");
    return static_cast<To>(from);

Clearly they're all runtime_errors, but should we be more specific?

Vadim, what would you say to a proposal to change those to:
        throw std::range_error("Cast would convert negative to unsigned.");
        throw std::underflow_error("Cast would transgress lower limit.");
        throw std::overflow_error("Cast would transgress upper limit.");
?