Distinguish between too many open connections and files.

These conditions are not exactly equivalent. And if anyone hits them,
we'd like to know which one actually happened.

Change-Id: I7e84916acde75560bd63bbe61f63dcf86324c9e7
Reviewed-on: http://review.couchbase.org/34276
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>