Better handling of VersionConflict

[blueyed]

I am seeing the following error (end of traceback), when running py.test:

  File "…/pytest/_pytest/config.py", line 917, in _preparse
    self.pluginmanager.load_setuptools_entrypoints("pytest11")
  File "…/pluggy/pluggy.py", line 501, in load_setuptools_entrypoints
    plugin = ep.load()
  File "…/pyenv/velodrome/lib/python3.4/site-packages/pkg_resources/__init__.py", line 2354, in load
    self.require(*args, **kwargs)
  File "…/pyenv/velodrome/lib/python3.4/site-packages/pkg_resources/__init__.py", line 2371, in require
    items = working_set.resolve(reqs, env, installer)
  File "…/pyenv/velodrome/lib/python3.4/site-packages/pkg_resources/__init__.py", line 844, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.VersionConflict: (coverage 4.0b3 (…/pyenv/velodrome/lib/python3.4/site-packages), Requirement.parse('coverage<4.0,>=3.7.1'))

The exception does not provide a hint about the problematic plugin.