Extend dbus.el by error messages, and :write access type

Extend dbus.el by error messages, and :write access type

* doc/misc/dbus.texi (Receiving Method Calls): Describe how to
produce D-Bus error messages.
(Receiving Method Calls): Support :write access type.

* lisp/net/dbus.el (dbus-error-dbus, dbus-error-failed)
(dbus-error-access-denied, dbus-error-invalid-args)
(dbus-error-property-read-only): New defconsts.
(dbus-method-error-internal): Add arg ERROR-NAME.
(dbus-register-method): Adapt docstring.
(dbus-handle-event): Handle error messages returned from the handler.
(dbus-get-this-registered-property)
(dbus-get-other-registered-property): New defuns.
(dbus-register-property): Support :write access type.
(dbus-property-handler): Submit proper D-Bus error messages.
Handle several paths at the same interface.

* src/dbusbind.c (Fdbus_message_internal): Improve handling of
DBUS_MESSAGE_TYPE_ERROR.