Handle EPIPE IOErrors when using more than 1 job

If someone is using flake8 and piping it to a command like head, the command they are piping flake8's output too may close the pipe earlier than flake8 expects. To avoid extraneous exception output being printed, we now catch IOErrors and check their errnos to ensure they're something we know we can ignore.

This also provides flexibility to add further errnos for ignoring on a case-by-case basis.

Closes #69 (closed)