More work on D-Bus error messages

More work on D-Bus error messages

* lisp/net/dbus.el (dbus-get-property): Adapt docstring.
(dbus-set-property): Handle case of `:write' access type.
(dbus-get-other-registered-properties): Rename from
`dbus-get-other-registered-property'.
(dbus-property-handler): Fix thinkos.

* src/dbusbind.c (xd_read_message_1): Add error_name to event args
in case of DBUS_MESSAGE_TYPE_ERROR.

* test/lisp/net/dbus-tests.el (dbus--test-enabled-session-bus)
(dbus--test-enabled-system-bus): Make them defconst.
(dbus--test-service, dbus--test-path, dbus--test-interface):
New defconst.  Replace all occurences of `dbus-service-emacs' by
`dbus--test-service'.
(dbus--test-method-handler): New defun.
(dbus-test04-register-method, dbus-test05-register-property): New tests.