Taxonomy: don't log error messages in case of invalid locale

In cases where document locale cannot be parsed (org.onehippo.taxonomy.util.TaxonomyUtil#toLocale ),
error message is logged:

log.error("Locale \"{}\" is not valid.", localeString);

This is annoying if you are using non-translated folders and default template prototype value stays unchanged ("document-type-locale").
We should lower this to a warning level at least.

NOTE: a workaround is to use a valid locale in prototype templates