fix error org.freedesktop.DBus.Error.ServiceUnknow

Gio.DBusError: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name im.pidgin.purple.PurpleService was not provided by any .service files

use try to catch error and ignore the following code

indeed shouldn't do many code in enable function!!

this may fix "Can't connect to Pidgin DBus events if Pidgin is not running. Restart Gnome Shell after launching Pidgin"

Signed-off-by: xiehuc <xiehuc@gmail.com>