plans: Update error message for apply validation (#21312)

* Update error message for apply validation

Add a hint that the validation failure has occurred at the root of the resource
schema to the error message. This is because the root resource has an empty
path when being validated and the path is being relied upon to provide context
into the error message.

plans/objchange/compatible.go

@@ -28,12 +28,17 @@ func AssertObjectCompatible(schema *configschema.Block, planned, actual cty.Valu
 
 func assertObjectCompatible(schema *configschema.Block, planned, actual cty.Value, path cty.Path) []error {
 	var errs []error
+	var atRoot string
+	if len(path) == 0 {
+		atRoot = "Root resource "
+	}
+
 	if planned.IsNull() && !actual.IsNull() {
-		errs = append(errs, path.NewErrorf("was absent, but now present"))
+		errs = append(errs, path.NewErrorf(fmt.Sprintf("%swas absent, but now present", atRoot)))
 		return errs
 	}
 	if actual.IsNull() && !planned.IsNull() {
-		errs = append(errs, path.NewErrorf("was present, but now absent"))
+		errs = append(errs, path.NewErrorf(fmt.Sprintf("%swas present, but now absent", atRoot)))
 		return errs
 	}
 	if planned.IsNull() {

terraform/eval_apply.go

@@ -245,7 +245,7 @@ func (n *EvalApply) Eval(ctx EvalContext) (interface{}, error) {
 						tfdiags.Error,
 						"Provider produced inconsistent result after apply",
 						fmt.Sprintf(
-							"When applying changes to %s, provider %q produced an unexpected new value for %s.\n\nThis is a bug in the provider, which should be reported in the provider's own issue tracker.",
+							"When applying changes to %s, provider %q produced an unexpected new value: %s.\n\nThis is a bug in the provider, which should be reported in the provider's own issue tracker.",
 							absAddr, n.ProviderAddr.Provider.String(), tfdiags.FormatError(err),
 						),
 					))