Add error message support to railgun

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.

This PR is the MSF side of https://github.com/rapid7/meterpreter/pull/26

lib/rex/post/meterpreter/extensions/stdapi/railgun/dll.rb

@@ -269,13 +269,21 @@ class DLL
     rec_out_only_buffers = response.get_tlv_value(TLV_TYPE_RAILGUN_BACK_BUFFERBLOB_OUT)
     rec_return_value = response.get_tlv_value(TLV_TYPE_RAILGUN_BACK_RET)
     rec_last_error = response.get_tlv_value(TLV_TYPE_RAILGUN_BACK_ERR)
+    rec_err_msg = response.get_tlv_value(TLV_TYPE_RAILGUN_BACK_MSG)
+
+    # Error messages come back with trailing CRLF, so strip it out
+    # if we do get a message.
+    rec_err_msg.strip! if not rec_err_msg.nil?
 
     #puts "received stuff"
     #puts "out_only_layout:"
     #puts out_only_layout
 
     # The hash the function returns
-    return_hash={"GetLastError" => rec_last_error}
+    return_hash = {
+      "GetLastError" => rec_last_error,
+      "ErrorMessage" => rec_err_msg
+    }
 
     #process return value
     case function.return_type

lib/rex/post/meterpreter/extensions/stdapi/railgun/tlv.rb

@@ -52,4 +52,5 @@ TLV_TYPE_RAILGUN_MEM_DATA    =          TLV_META_TYPE_RAW   | (TLV_TYPE_EXTENSIO
 TLV_TYPE_RAILGUN_MEM_LENGTH            =  TLV_META_TYPE_UINT   | (TLV_TYPE_EXTENSION_RAILGUN + TLV_EXTENSIONS + 14)
 
 TLV_TYPE_RAILGUN_CALLCONV              = TLV_META_TYPE_STRING  | (TLV_TYPE_EXTENSION_RAILGUN + TLV_EXTENSIONS + 15)
+TLV_TYPE_RAILGUN_BACK_MSG              = TLV_META_TYPE_STRING  | (TLV_TYPE_EXTENSION_RAILGUN + TLV_EXTENSIONS + 16)
 end; end; end; end; end; end