In case of exception we return an html page in case the client is a browser

apps/dav/lib/connector/sabre/objecttree.php

apps/dav/lib/connector/sabre/serverfactory.php

apps/dav/lib/files/browsererrorpageplugin.php

apps/dav/lib/server.php

apps/dav/templates/exception.php

apps/dav/tests/unit/dav/browsererrorpageplugintest.php

lib/private/template.php