slapos/testing: make the error message more comprehensible

See merge request !366
8 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

01:38:31

failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python3

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failed SlapOS-Master.UnitTest-Master

01:40:53

failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python3

00:14:57

failed SlapOS.Eggs.UnitTest-Master.Python3

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