Ensure the error responsible for API failures is always added

[Martchus]

So far the "last error" was only tracked for REST-API calls (used e.g. for worker registration and uploads) but not in other cases (e.g. web socket related errors). This change extends tracking for to cover those cases by setting the last error to the error messages that is always passed to _set_status. This way they are available to be added to the reason when declaring the job as incomplete.

Related ticket: https://progress.opensuse.org/issues/163781

[kalikiana]

[15:02:28] t/ui/10-tests_overview.t ................... 10/? 
        #   Failed test '#flavor_DVD_arch_i586 present exactly once'
        #   at /home/squamata/project/t/ui/../lib/OpenQA/SeleniumTest.pm line 237.
        #          got: '0'
        #     expected: '1'

        #   Failed test '#flavor_DVD_arch_i586 exists'
        #   at /home/squamata/project/t/ui/../lib/OpenQA/SeleniumTest.pm line 240.
        # Looks like you failed 2 tests of 8.

    #   Failed test 'filter for specific archs'
    #   at t/ui/10-tests_overview.t line 259.
    # Looks like you failed 1 test of 2.

[perlpunk]

https://progress.opensuse.org/issues/166820 t/ui/10-tests_overview.t

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.73%. Comparing base (b54d8ce) to head (d5f4359).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5927      +/-   ##
==========================================
- Coverage   98.73%   98.73%   -0.01%     
==========================================
  Files         395      395              
  Lines       38766    38768       +2     
==========================================
- Hits        38277    38276       -1     
- Misses        489      492       +3     

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