Character backend open hasn't been fully converted to the Error API.
Some opens fail without setting an error.  qmp_chardev_add() needs to
detect when that happens, and set a generic error.  Explain that in a
comment, and inline error_is_set() for clarity.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>