Hint about a possible update failure reason

[strugee]

This feels ugly because it doesn't fix the underlying bug, but
it's incredibly hard to trace down the root cause in this
situation and it causes updates to fail which is pretty
critical. So this is probably better than nothing.

Ref QubesOS/qubes-issues#5052

[codecov]

Codecov Report

Merging #225 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #225   +/-   ##
=======================================
  Coverage   72.65%   72.65%           
=======================================
  Files           3        3           
  Lines         512      512           
=======================================
  Hits          372      372           
  Misses        140      140           

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[marmarek]

IMO better than guessing, make the actual error easier to find. It should be written to /var/lib/qubes/updates/errors in dom0, so qubes-dom0-update script may check this file and show it to the user. If that still doesn't contain enough details, you can modify qubes-receive-updates script to save stderr of qfile-dom0-unpacker there too.