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* BB Pull Request 126: DistributionNotFound message now lists the package or
packages that required it. E.g.::
pkg_resources.DistributionNotFound: The 'colorama>=0.3.1' distribution was not found and is required by smlib.log.
Note that zc.buildout once dependended on the string rendering of this
message to determine the package that was not found. This expectation
has since been changed, but older versions of buildout may experience
problems. See Buildout 242 for details.