Add debug message for cluster unreachable at plan time (#962)

The CustomizeDiffFunc function doesn't allow us to surface
diagnostic messages when something goes wrong, only errors.

We don't want the plan to fail so instead we mark "manifest"
as computed and swallow the error, and write a debug log.

helm/resource_release.go

@@ -862,6 +862,7 @@ func resourceDiff(ctx context.Context, d *schema.ResourceDiff, meta interface{})
 					// NOTE it would be nice to return a diagnostic here to warn the user
 					// that we can't generate the diff here because the cluster is not yet
 					// reachable but this is not supported by CustomizeDiffFunc
+					debug(`cluster was unreachable at create time, marking "manifest" as computed`)
 					return d.SetNewComputed("manifest")
 				}
 				return err