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.

(cherry picked from commit 105503b4b470c124bc0c271bd8a50e25ecbe9133)
(removed the buffering of warnings in aptwarnings.log, as we do not
 have a cleanup function where we can cat it)

LP: #1642386

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