added changes for 412 error handling

[DO-rrao]

Before the fix:

doctl registry repository list-v2
# Error: failed to get registry: GET .../v2/registry: 412 (opaque error)

After the fix:

doctl registry repository list-v2
# Error: multiple registries found. Please specify one with --registry <name>.
# Available registries: registry1, registry2, registry3

# User now knows what to do:
doctl registry repository list-v2 --registry registry1