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!!

Signed-off-by: xiehuc <xiehuc@gmail.com>
Merge pull request #9 from xiehuc/master

fix error org.freedesktop.DBus.Error.ServiceUnknow