Improve messages of string_generator

[ivantishchenko]

It would be nice to extend the exception message for
https://github.com/boostorg/uuid/blob/develop/include/boost/uuid/string_generator.hpp

For example:

• "" => "invalid uuid string ''. uuid cannot be empty'
• "123------123" => "invalid uuid string '123------123'. uuid cannot contain consecutive dashes'.
• ...

[pdimov]

The exception messages have been improved a bit, although they still don't really match what you suggest. "" now says "Invalid UUID string at position 0: unexpected end of input", whereas "123------123" says "Invalid UUID string at position 3: hex digit expected".