cmd/dirserver/main.go: log nonfatal if signature fails
cmd/dirserver/main.go: log nonfatal if signature fails

Warn but don't die if signature fails to verify, e.g. because there are
no users loaded yet.

Change-Id: I6ff4963990a5dee7d7f735921797ec7e0196706b
Reviewed-on: https://upspin-review.git.corp.google.com/1927
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>

upspin/upspin,upspin/upspin