web app: nicer error message about non-existent origins
Consider https://archive.softwareheritage.org/browse/origin/https://github.com/bic-mni/mni_autoreg/directory/
The returned error message reads "'NoneType' object does not support item assignment".
Rather, it shoulds say something about the fact that the requested origin does not exist in the archive.
Migrated from T1109 (view on Phabricator)