report apt-key errors via status-fd messages

report apt-key errors via status-fd messages

We report warnings from apt-key this way already since
29c590951f812d9e9c4f17706e34f2c3315fb1f6, so reporting errors seems like
a good addition. Most of those errors aren't really from apt-key
through, but from the code setting up and actually calling it which used
to just print to stderr which might or might not intermix them with
(other) progress lines in update calls. Having them as proper error
messages in the system means that the errors are actually collected
later on for the list instead of ending up with our relatively generic
but in those cases bogus hint regarding "is gpgv installed?".

The effective difference is minimal as the errors apply mostly to
systems which have far worse problems than a not as nice looking error
message, which makes this pretty hard to test – but at least now the
hint that your system is broken can be read in proper order (= there
aren't many valid cases in which the permissions of /tmp are messed up…).

LP: #1522988