Fetching a default namespace provider from the public registry can
result in 404 Not Found error. This might be caused by a previously-
default provider moving to a new namespace, which means that the
configuration needs to be upgraded to use an explicit provider source.
This commit adds a more detailed diagnostic for this situation,
suggesting that the intended provider might be in a new namespace. The
recommended course of action is to run the 0.13upgrade command to
generate the correct required_providers configuration.
Alisdair McDiarmid