demo-app: Removed try block
demo-app: Removed try block

Removed try block since action_register_callback should no longer raise an
exception, just return a status code. Also change all instance of error_string()
to status_string().

Signed-off-by: Ben Astell <ded3fae2663d1648fc922f7692e7914ea175e4a7@windriver.com>

rcwoolley/device-cloud-python,rcwoolley/device-cloud-python