[Buildbot] #2875: support retryOnFailure

[Buildbot trac]

#2875: support retryOnFailure
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Reporter:  dustin       |       Owner:
    Type:  enhancement  |      Status:  new
Priority:  major        |   Milestone:  0.9.0
 Version:  0.8.9        |  Resolution:
Keywords:               |
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Comment (by sa2ajj):

 What is the reason for the virtualenv step to fail in the first place?

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Ticket URL: <http://trac.buildbot.net/ticket/2875#comment:1>
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