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

@@ -995,18 +995,19 b' def ValidAuthPlugins():'
             from rhodecode.authentication.base import loadplugin
             module_list = value
             unique_names = {}
-            try:
-                for module in module_list:
-                    plugin = loadplugin(module)
-                    plugin_name = plugin.name
-                    if plugin_name in unique_names:
-                        msg = M(self, 'import_duplicate', state,
-                                loaded=unique_names[plugin_name],
-                                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)
+            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,
+                            loaded=unique_names[plugin_name],
+                            next_to_load=plugin_name)
+                    raise formencode.Invalid(msg, value, state)
+                unique_names[plugin_name] = plugin
 
     return _validator