Silently recover if REDO_CHEATFDS file descriptors are closed, because
they aren't completely essential and MAKEFLAGS-related warnings already
get printed if all file descriptors have been closed.
If MAKEFLAGS --jobserver-auth flags are closed, improve the error
message so that a) it's a normal error instead of an exception and b)
we link to documentation about why it happens. Also write some more
detailed documentation about what's going on here.
