add custom HTTP error pages for swh-web

swh/web/browse/urls.py

swh/web/browse/views/content.py

swh/web/browse/views/directory.py

swh/web/browse/views/origin.py

swh/web/browse/views/person.py

swh/web/browse/views/revision.py

swh/web/common/exc.py

swh/web/static/css/style.css

swh/web/static/img/swh-logo.svg

swh/web/templates/error.html

swh/web/tests/browse/views/test_content.py

swh/web/tests/browse/views/test_directory.py

swh/web/tests/browse/views/test_origin.py

swh/web/tests/browse/views/test_person.py

swh/web/tests/browse/views/test_revision.py

swh/web/urls.py