Handle VersionConflict in load_setuptools_entrypoints

[blueyed]

This is an attempt at addressing #7.

It might get handled better, e.g. in pytest itself, but this makes it at least be more verbose and mentions the source of the error (the plugin causing it).

Traceback (most recent call last):
  File "…/pluggy/pluggy.py", line 502, in load_setuptools_entrypoints
    plugin = ep.load()
  File "…/pyenv/project/lib/python3.4/site-packages/pkg_resources/__init__.py", line 2354, in load
    self.require(*args, **kwargs)
  File "…/pyenv/project/lib/python3.4/site-packages/pkg_resources/__init__.py", line 2371, in require
    items = working_set.resolve(reqs, env, installer)
  File "…/pyenv/project/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/project/lib/python3.4/site-packages), Requirement.parse('coverage<4.0,>=3.7.1'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "…/pyenv/project/bin/py.test", line 9, in <module>
    load_entry_point('pytest==2.7.3.dev410+ng8625eb6', 'console_scripts', 'py.test')()
  File "…/pytest/_pytest/config.py", line 38, in main
    config = _prepareconfig(args, plugins)
  File "…/pytest/_pytest/config.py", line 117, in _prepareconfig
    pluginmanager=pluginmanager, args=args)
  File "…/pluggy/pluggy.py", line 728, in __call__
    return self._hookexec(self, self._nonwrappers + self._wrappers, kwargs)
  File "…/pluggy/pluggy.py", line 338, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "…/pluggy/pluggy.py", line 333, in <lambda>
    _MultiCall(methods, kwargs, hook.spec_opts).execute()
  File "…/pluggy/pluggy.py", line 599, in execute
    return _wrapped_call(hook_impl.function(*args), self.execute)
  File "…/pluggy/pluggy.py", line 249, in _wrapped_call
    wrap_controller.send(call_outcome)
  File "…/pytest/_pytest/helpconfig.py", line 28, in pytest_cmdline_parse
    config = outcome.get_result()
  File "…/pluggy/pluggy.py", line 278, in get_result
    raise ex[1].with_traceback(ex[2])
  File "…/pluggy/pluggy.py", line 264, in __init__
    self.result = func()
  File "…/pluggy/pluggy.py", line 600, in execute
    res = hook_impl.function(*args)
  File "…/pytest/_pytest/config.py", line 851, in pytest_cmdline_parse
    self.parse(args)
  File "…/pytest/_pytest/config.py", line 956, in parse
    self._preparse(args)
  File "…/pytest/_pytest/config.py", line 917, in _preparse
    self.pluginmanager.load_setuptools_entrypoints("pytest11")
  File "…/pluggy/pluggy.py", line 507, in load_setuptools_entrypoints
    "Plugin %r could not be loaded: %s!" % (ep.name, e))
pluggy.PluginValidationError: Plugin 'testmon' could not be loaded: (coverage 4.0b3 (…/pyenv/project/lib/python3.4/site-packages), Requirement.parse('coverage<4.0,>=3.7.1'))!
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