Handle VersionConflict in load_setuptools_entrypoints

Ref: #7

pluggy.py

@@ -492,7 +492,8 @@ def check_pending(self):
     def load_setuptools_entrypoints(self, entrypoint_name):
         """ Load modules from querying the specified setuptools entrypoint name.
         Return the number of loaded plugins. """
-        from pkg_resources import iter_entry_points, DistributionNotFound
+        from pkg_resources import (iter_entry_points, DistributionNotFound,
+                                   VersionConflict)
         for ep in iter_entry_points(entrypoint_name):
             # is the plugin registered or blocked?
             if self.get_plugin(ep.name) or self.is_blocked(ep.name):
@@ -501,6 +502,9 @@ def load_setuptools_entrypoints(self, entrypoint_name):
                 plugin = ep.load()
             except DistributionNotFound:
                 continue
+            except VersionConflict as e:
+                raise PluginValidationError(
+                    "Plugin %r could not be loaded: %s!" % (ep.name, e))
             self.register(plugin, name=ep.name)
             self._plugin_distinfo.append((plugin, ep.dist))
         return len(self._plugin_distinfo)