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.

terraform/eval_apply.go

@@ -158,6 +158,7 @@ func (n *EvalApply) Eval(ctx EvalContext) (interface{}, error) {
 	// we still want to save that but it often causes some confusing behaviors
 	// where it seems like Terraform is failing to take any action at all,
 	// so we'll generate some errors to draw attention to it.
+	if !applyDiags.HasErrors() {
 		if change.Action == plans.Delete && !newVal.IsNull() {
 			diags = diags.Append(tfdiags.Sourceless(
 				tfdiags.Error,
@@ -178,6 +179,7 @@ func (n *EvalApply) Eval(ctx EvalContext) (interface{}, error) {
 				),
 			))
 		}
+	}
 
 	var newState *states.ResourceInstanceObject
 	if !newVal.IsNull() { // null value indicates that the object is deleted, so we won't set a new state in that case