upgrade-provider swallows errors from make upstream

[thomas11]

> upgrade-provider pulumi/pulumi-azure --kind=all
[...]
Applied patch to 'internal/services/web/function_app_slot_resource.go' cleanly.
U internal/services/firewall/firewall_policy_resource.go
Recorded preimage for 'internal/services/firewall/firewall_policy_resource.go'
make: *** [upstream] Error 1

Whereas

> sh ./scripts/upstream.sh upstream apply
Applied patch to 'internal/services/web/function_app_slot_resource.go' cleanly.
U internal/services/firewall/firewall_policy_resource.go
Recorded preimage for 'internal/services/firewall/firewall_policy_resource.go'

make "upstream"' failed to apply a patch. This is because there is a conflict between
the checked out version of upstream and the patch set. To resolve this conflict
run:

    FROM=$LAST_KNOWN_GOOD_COMMIT make upstream.rebase

This will walk you through resolving the conflict and producing a patch set that
cleanly applies to the current upstream.