Improve invalid fd error messages for epoll

[erikgeiser]

Previously, the error returned by unix.EpollCtl was discarded. This PR improves the error messages and includes the file descriptors as they are likely related to any error that can occur. Additionally a -1 file descriptor is caught early with a descriptive error message.

[Blane245]

I am pursuing a problem that I am having with go.d.plugin and it is right here. I would suggest that the code not substitute another message has you have done, but use the actual cancelreader error codes.

BTW, my problem is that go.d.plugin is not collecting when it is spawned by my server but will work when run from the console or a shell script. The only difference in the logs this error message. The problem started a couple of days ago.