gmpxx.h constructor exceptions

[Kevin Ryde]

When the mpz_class(string) constructor or operator= finds its string
is not a valid number, I'm thinking to have it throw
std::invalid_argument.  This is the same as what std::bitset does, if
I read it right.  Does that sound sensible to C++ experts lurking
here?

And for the gmp_randclass constructor using gmp_randinit_lc_2exp_size
I'm thinking of std::length_error for a size bigger than the lc_2exp
table data supports.  This would be like the std::string routines
throw when asked to exceed their max_size.