correctly recognize bad LDAP credentials in test

- also raise when testing connection of an abstract auth source, avoiding wrong success messages

app/models/auth_source.rb

@@ -40,7 +40,9 @@ class AuthSource < ActiveRecord::Base
   def authenticate(_login, _password)
   end
 
+  # implemented by a subclass, should raise when no connection is possible and not raise on success
   def test_connection
+    raise I18n.t('auth_source.using_abstract_auth_source')
   end
 
   def auth_method_name

app/models/ldap_auth_source.rb

@@ -53,10 +53,11 @@ class LdapAuthSource < AuthSource
 
   # test the connection to the LDAP
   def test_connection
-    ldap_con = initialize_ldap_con(account, account_password)
-    ldap_con.open {}
+    unless authenticate_dn(account, account_password)
+      raise I18n.t('auth_source.ldap_error', error_message: I18n.t('auth_source.ldap_auth_failed'))
+    end
   rescue  Net::LDAP::LdapError => text
-    raise 'LdapError: ' + text.to_s
+    raise I18n.t('auth_source.ldap_error', error_message: text.to_s)
   end
 
   def auth_method_name

config/locales/en.yml

@@ -1091,6 +1091,11 @@ en:
   label_keyboard_shortcut_focus_next_item: "Focus next list element (on some lists only)"
   label_visible_elements: Visible elements
 
+  auth_source:
+    using_abstract_auth_source: "Can't use an abstract authentication source."
+    ldap_error: "LDAP-Error: %{error_message}"
+    ldap_auth_failed: "Could not authenticate at the LDAP-Server."
+
   macro_execution_error: "Error executing the macro %{macro_name}"
   macro_unavailable: "Macro %{macro_name} cannot be displayed."