Improve PluginValidationErrors

[blueyed]

PluginValidationErrors should be turned into a friendlier error message, and especially stating the reason in a more useful way.

pluggy.manager.PluginValidationError: Plugin 'xdist' could not be loaded: (pytest 4.1.1 (/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages), Requirement.parse('pytest>=4.4.0'))!

This should say instead:

The plugin "xdist" could not be loaded (requires pytest>=4.4.0, but has pytest 4.1.1)

The best might be to check/handle PluginValidationError in main().

Currently it looks like this:

Traceback (most recent call last):
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/pluggy/manager.py", line 274, in load_setuptools_entrypoints
    plugin = ep.load()
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2433, in load
    self.require(*args, **kwargs)
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2456, in require
    items = working_set.resolve(reqs, env, installer, extras=self.extras)
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/pkg_resources/__init__.py", line 791, in resolve
    raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.VersionConflict: (pytest 4.1.1 (/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages), Requirement.parse('pytest>=4.4.0'))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/bin/py.test", line 10, in <module>
    sys.exit(main())
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/_pytest/config/__init__.py", line 61, in main
    config = _prepareconfig(args, plugins)
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/_pytest/config/__init__.py", line 196, in _prepareconfig
    pluginmanager=pluginmanager, args=args
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/pluggy/hooks.py", line 289, in __call__
    return self._hookexec(self, self.get_hookimpls(), kwargs)
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/pluggy/manager.py", line 68, in _hookexec
    return self._inner_hookexec(hook, methods, kwargs)
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/pluggy/manager.py", line 62, in <lambda>
    firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/pluggy/callers.py", line 203, in _multicall
    gen.send(outcome)
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/_pytest/helpconfig.py", line 93, in pytest_cmdline_parse
    config = outcome.get_result()
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/pluggy/callers.py", line 80, in get_result
    raise ex[1].with_traceback(ex[2])
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/pluggy/callers.py", line 187, in _multicall
    res = hook_impl.function(*args)
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/_pytest/config/__init__.py", line 652, in pytest_cmdline_parse
    self.parse(args)
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/_pytest/config/__init__.py", line 838, in parse
    self._preparse(args, addopts=addopts)
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/_pytest/config/__init__.py", line 784, in _preparse
    self.pluginmanager.load_setuptools_entrypoints("pytest11")
  File "/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages/pluggy/manager.py", line 280, in load_setuptools_entrypoints
    message="Plugin %r could not be loaded: %s!" % (ep.name, e),
pluggy.manager.PluginValidationError: Plugin 'xdist' could not be loaded: (pytest 4.1.1 (/tmp/tox…/Vcs/pytest-testmon/py36-pytest41-xdist/lib/python3.6/site-packages), Requirement.parse('pytest>=4.4.0'))!

Somehow related PR for inspiration: #4077.