ipc: more helpful ETOOMANYREFS error messages

ipc: more helpful ETOOMANYREFS error messages

ETOOMANYREFS is probably a unfamiliar error to most users, so
give a hint about RLIMIT_NOFILE.  This can be hit on my system
running 3 simultaneous queries with my system default limit of
1024.

There's also no need to import Errno constants for uncommon
errors, so we'll stop using Errno, here.

We'll also try to bump RLIMIT_NOFILE as much as possible
to avoid this error.