Log ErrUnexpectedEOF from watches as warnings

Watches are often established via long-running HTTP GET requests which
will inevitably time out during the normal course of operations. When
the watches time out, an io.EOF or an io.ErrUnexpectedEOF will bubble
up to client components such as StreamWatcher and Reflector. Treat EOF
as a clean watch termination. Treat ErrUnexpectedEOF as a less-clean
but non-fatal watch termination and log the event at the warning level.

This greatly reduces the amount of log noise generated during what is
ultimately normal operation, and adds the flexibility for the operator
to make a distinction between the EOF conditions if so desired (by
adjusting the logging level).
Merge pull request #2767 from ironcladlou/watch-error-handling

Log ErrUnexpectedEOF from watches as warnings