[Buildbot] #3556: Failed FileUpload step doesn't show error (file not found)

[Buildbot trac]

#3556: Failed FileUpload step doesn't show error (file not found)
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Reporter:  rutsky     |      Owner:
    Type:  undecided  |     Status:  new
Priority:  major      |  Milestone:  0.9.+
 Version:  0.9.0b9    |   Keywords:
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 If `steps.FileUpload` step fails due to missing file, no meaningful error
 is displayed in the UI.

 Attaching screenshot of such failed build.

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Ticket URL: <http://trac.buildbot.net/ticket/3556>
Buildbot <http://buildbot.net/>
Buildbot: build/test automation