Add a test verifying that attempting to add a nonexistent attribute to
ignore_changes throws an error.

Return a warning if a Computed attribute is present in ignore_changes,
unless the attribute is also Optional.

ignore_changes on a non-Optional Computed attribute is a no-op, so the user
likely did not want to set this in config.
An Optional Computed attribute, however, is still subject to ignore_changes
behaviour, since it is possible to make changes in the configuration that
Terraform must ignore.