fix(cpp-client): Fix certain error messages

[kosak]

The string concatenation was happening in the wrong place, so the original code was producing messages like

due to unexpected exception with message:
  gethostname failed: error code : class std::basic_string<char,struct std::
  char_traits<char>,class std::allocator<char> > __cdecl deephaven::dhcore::
  utility::GetHostname(void)@C:\Users\kosak\git\deephaven-core\cpp-client\
  deephaven\dhcore\src\utility\utility_platform_specific.cc:6710093

In the above the Windows error code is "10093" and the line number is "67".

The original code was mashing together both into 6710093