docker: fail more gracefully on docker.py check

As this is called directly from the Makefile while determining
dependencies and it is possible the user was configured in one window
but not have credentials in the other. Let's catch the Exceptions and
deal with it quietly.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>