authn: Fix form validation of authentication plugins.

- Check if plugin exists.
- Display an apropriate error message if it doesn't exist.

rhodecode/authentication/base.py

@@ -474,12 +474,7 b' def loadplugin(plugin_id):'
     """
     Loads and returns an instantiated authentication plugin.
     Returns the RhodeCodeAuthPluginBase subclass on success,
-    raises exceptions on failure.
-
-    raises:
-        KeyError -- if no plugin available with given name
-        TypeError -- if the RhodeCodeAuthPlugin is not a subclass of
-                     ours RhodeCodeAuthPluginBase
+    or None on failure.
     """
     # TODO: Disusing pyramids thread locals to retrieve the registry.
     authn_registry = get_current_registry().getUtility(IAuthnPluginRegistry)

rhodecode/model/validators.py

@@ -995,9 +995,12 b' def ValidAuthPlugins():'
             from rhodecode.authentication.base import loadplugin
             module_list = value
             unique_names = {}
-            try:
                 for module in module_list:
                     plugin = loadplugin(module)
+                if plugin is None:
+                    raise formencode.Invalid(
+                        _("Can't find plugin with id '{}'".format(module)),
+                        value, state)
                     plugin_name = plugin.name
                     if plugin_name in unique_names:
                         msg = M(self, 'import_duplicate', state,
@@ -1005,8 +1008,6 b' def ValidAuthPlugins():'
                                 next_to_load=plugin_name)
                         raise formencode.Invalid(msg, value, state)
                     unique_names[plugin_name] = plugin
-            except (KeyError, AttributeError, TypeError) as e:
-                raise formencode.Invalid(str(e), value, state)
 
     return _validator