Temporary fix: Avoid killing Coordinator with unregistered cache mgr actions

Temporary fix: Avoid killing Coordinator with unregistered cache mgr actions
that cause isOpen() assertions.

If a worker forwards a cache manager request to Coordinator and Coordinator
does not have that action registered, CacheManager::createRequestedAction()
throws (as it should) and Mgr::Request cleanup asserts when its half-baked
connection tries to close a not-yet-imported socket descriptor.

This workaround catches the exception, reports it, and manually closes the
socket descriptor. It also prevents an ACK response from being sent to the
worker, which triggers a worker timeout.

Mid-term TODO: Coordinator should register all actions that are known to kids.
Should Coordinator respond with an error instead of relying on a timeout?

Long-term TODO: Consider an API where cache manager responses can be
aggregated and formatted by Coordinator without knowing action-specific
details. After all, there are not so many types of action information (size,
count, rate, etc.) and most actions have simple reporting formats. Currently,
it is awkward to guarantee that Coordinator and all workers know all actions,
especially when some actions may be specific to non-worker kids such as
Coordinator and diskers.