process: Wake up read and write on `EPOLLERR` (#2218)

## Motivation

#2174

On epoll platforms, the read end of a pipe closing is signaled to the write end
through the `EPOLLERR` event [[1](http://man7.org/linux/man-pages/man2/epoll_ctl.2.html)]. If readiness is not registered for this
event, it will silently pass through `epoll_wait` calls.

Additionally, this specific case that `EPOLLERR` is triggered leaves the write
end of the pipe (parent process) waiting for a wakeup that never occurs.

## Solution

Similar to the `HUP` event on Unix platforms, errors are now always masked
through registrations so that both read and write ends of a connection are made
aware of errors.

In cases where pipes are used and the read end closes, write ends that are
waiting for a wakeup are properly notified and try to write again. This allows
a client to observe `BrokenPipe` and go through the proper cleanup and/or
restablishment of connection.

Closes #2174

Signed-off-by: Kevin Leimkuhler <kevin@kleimkuhler.com>

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