DistributionNotFound: Show requirers

It is very helpful to know who required the missing package. E.g.:

    pkg_resources.DistributionNotFound: The 'colorama>=0.3.1' distribution was not found and is required by smlib.log.

Note that there was a comment: "unfortunately, zc.buildout uses a str(err) to
get the name of the distribution here..", but I did a search in the `buildout`
code and I think that is no longer true, so I think that we're free to make the
exception message more helpful without risk of breaking buildout:

    # In clone from https://github.com/buildout/buildout
    $ ag DistributionNotFound
    src/zc/buildout/buildout.py
    686:        except (ImportError, pkg_resources.DistributionNotFound):

    $ pip install --download=. --no-install zc.buildout
    ...
      Saved ./zc.buildout-2.3.1.tar.gz
    ...
    $ tar xf zc.buildout-2.3.1.tar.gz
    $ ag DistributionNotFound zc.buildout-2.3.1
    zc.buildout-2.3.1/src/zc/buildout/buildout.py
    686:        except (ImportError, pkg_resources.DistributionNotFound):

--HG--
branch : DistributionNotFound_list_requirers

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