New strings to localization (#5225)

* error_can_not_delete_custom_field: Unable to delete custom field
* error_unable_to_connect: Unable to connect ({{value}})
* error_can_not_remove_role: This role is in use and can not be deleted.
* error_can_not_delete_tracker: This tracker contains issues and can't be deleted.


git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3627 e93f8b46-1217-0410-a6f0-8f06a7374b81