Skip a build if git clone fails

[vstinner]

Currently, when "git clone" fails, the whole build is marked as an error:
https://buildbot.python.org/all/#/builders/57/builds/266

• job 0 "git" failed because "git clone" failed (network issue)
• job 1 "make distclean" failed because there is no Makefile: this one is fine, Clean step uses warnOnFailure = 1, it's just a warning

I failed to find the proper configuration to say: if git fails, give up on this build, but don't mark is as an error.

[vstinner]

BuildStep parameters:
http://docs.buildbot.net/latest/manual/configuration/buildsteps.html#common-parameters-of-source-checkout-operations

The Git step uses haltOnFailure = True, config from the base class:

    # if the checkout fails, there's no point in doing anything else
    haltOnFailure = True
    flunkOnFailure = True
    notReally = False

https://github.com/buildbot/buildbot/blob/5433c07b04c8f6ab9b8e4de74f806fc96718ae8a/master/buildbot/steps/source/base.py#L47-L50

[vstinner]

My attempt to fix this issue: #69