Add error message support to the railgun code

This code was lost in the transition when the meterpreter source was
removed from the metasploit-framework source. I'm pulling this in by
request of @dmaloney-r7 who originally requested this code be inculded
as part of https://github.com/rapid7/metasploit-framework/pull/740

I added an extra bit of code to free up memory that is allocated by the
call to FormatMessage and forced the ASCII-version (FormatMessageA) of
the call.