apt-key: warn instead of fail on unreadable keyrings

apt-key has inconsistent behaviour if it can't read a keyring file:
Commands like 'list' skipped silently over such keyrings while 'verify'
failed hard resulting in apt to report cconfusing gpg errors (#834973).

As a first step we teach apt-key to be more consistent here skipping in
all commands over unreadable keyrings, but issuing a warning in the
process, which is as usual for apt commands displayed at the end of the
run.

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