plugins: replace PluginError with RuntimeError

Co-authored-by: dgw <dgw@technobabbl.es>

sopel/plugins/__init__.py

@@ -69,7 +69,7 @@ def find_internal_plugins():
     """
     modules = importlib.util.find_spec('sopel.modules')
     if modules is None or modules.submodule_search_locations is None:
-        raise exceptions.PluginError('Cannot resolve internal plugins')
+        raise RuntimeError('Cannot resolve internal plugins')
     plugin_list = itertools.chain.from_iterable(
         _list_plugin_filenames(path)
         for path in modules.submodule_search_locations