UIDGenerator: better support for 32-bit machines

UIDGenerator: better support for 32-bit machines

Replace one exception with another that is less likely. Integer
multiplication is exact up to the limit of the double precision
mantissa size, and wfBaseConvert() will work up to any size of string.
So it's not necessary to require bcmath/gmp to be installed. The modulo
operation is equivalent to truncation of the binary form.

Perhaps mantissa sizes will be extended before the year 144680, made
possible by improved computing hardware. The exception message should be
altered when such hardware becomes available.

Change-Id: I8116dac1958b5dbef977d3900973bf13be3d9d17