Fix newly introduced error in handler for forward ECONNREFUSED

Created by: maartenth

In my merged PR #1099, I was too fast. I didn't make enough changes in the error handler and for ECONNREFUSED this introduced a new error, since proxyReq was not defined.

To fix it, I wrapped the error handler in a createErrorHandler() function to specify which request should be handled (proxyReq or forwardReq).