docker: fix error message formatting; make exception python3 compat
docker: fix error message formatting; make exception python3 compat

Ali-Razmjoo/zaproxy,meitar/zaproxy,Ali-Razmjoo/zaproxy,zaproxy/zaproxy,kingthorin/zaproxy,meitar/zaproxy,thc202/zaproxy,zaproxy/zaproxy,meitar/zaproxy,zaproxy/zaproxy,zapbot/zaproxy,zaproxy/zaproxy,psiinon/zaproxy,gmaran23/zaproxy,thc202/zaproxy,zapbot/zaproxy,zaproxy/zaproxy,gmaran23/zaproxy,gsavastano/zaproxy,zapbot/zaproxy,meitar/zaproxy,meitar/zaproxy,Ali-Razmjoo/zaproxy,meitar/zaproxy,psiinon/zaproxy,zaproxy/zaproxy,Ali-Razmjoo/zaproxy,psiinon/zaproxy,gmaran23/zaproxy,Ali-Razmjoo/zaproxy,psiinon/zaproxy,gmaran23/zaproxy,thc202/zaproxy,zapbot/zaproxy,kingthorin/zaproxy,kingthorin/zaproxy,gmaran23/zaproxy,psiinon/zaproxy,gsavastano/zaproxy,kingthorin/zaproxy,zapbot/zaproxy,gsavastano/zaproxy,zapbot/zaproxy,gsavastano/zaproxy,thc202/zaproxy,gsavastano/zaproxy,Ali-Razmjoo/zaproxy,thc202/zaproxy,zapbot/zaproxy,gsavastano/zaproxy,psiinon/zaproxy,gmaran23/zaproxy,psiinon/zaproxy,gmaran23/zaproxy,zaproxy/zaproxy,meitar/zaproxy,kingthorin/zaproxy,kingthorin/zaproxy,kingthorin/zaproxy,thc202/zaproxy,Ali-Razmjoo/zaproxy,meitar/zaproxy,thc202/zaproxy,gsavastano/zaproxy