Slight innacuracy in `ip_address` `try_value_to` example

```
boost::system::result< unsigned char > oct1
= try_value_to< unsigned char >( arr[0] );
if( !oct1 )
return make_error_code( std::errc::invalid_argument );
```
This should be
```
if( !oct1 )
return oct1.error();
```
so as to not discard the error returned from the nested `try_value_to` (which is going to be more informative and may include a source location).