Report serverListener_tcp() errors more clearly

[john8329]

I'm trying to run the system on Windows, and the way it uses to connect to plugins is TCP. I see that the functions refer to two environment variables, PLUGIN_MIN_PORT and PLUGIN_MAX_PORT. If they are undefined, a clueless error is returned.

I kind of understand why they have to be declared as environment variables, it makes good sense, though the error took me some time to find out what it actually was. Please report the absence of these variables more clearly. Or make it configurable in the code.

Thanks

[mitchellh]

This is fixed by #111!