serverutil/frontend: handle directory server failures
serverutil/frontend: handle directory server failures

The watch loop wasn't handling the case where the dir server went away.
I guess it's a testament to how stable the directory server has been,
that we didn't notice this until now.

Change-Id: Ic31711841f575aff3eb0e8413dab1c91f725fa0c
Reviewed-on: https://upspin-review.googlesource.com/c/upspin/+/19660
Reviewed-by: 395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

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