core: Generate "provider bug" warnings only if no errors from provider
core: Generate "provider bug" warnings only if no errors from provider

We can be more relaxed about our rules that a create musn't return null
or a destroy must return null if the provider also itself indicated an
error. In that case, it's expected that the return value is describing a
partial result, and so we'll just store it and move on.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk