Avoid workers from showing up shortly as broken after registration

[Martchus]

When working on https://progress.opensuse.org/issues/154927 I noticed that we don't reset the error state of a worker on registration. Normally this is not notable because we do when the worker sends a status update which happens only shortly after the registration. However, it makes still sense to clear the old error status on the registration to avoid workers from possibly being considered broken in the small window between the registration and the first status update. This makes only a difference when the worker did not disconnect gracefully.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0d50a81) 98.37% compared to head (28bf0ed) 98.37%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5451   +/-   ##
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  Coverage   98.37%   98.37%           
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  Files         389      389           
  Lines       37751    37754    +3     
=======================================
+ Hits        37139    37142    +3     
  Misses        612      612           

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