getting form errors is inconsistent

If you have a form element with the #parents 1, 2 and 3 setting form errors would work when you set it on either "1" or "1][2][3". Thus form_get_error() checks the first parent or all parents, but it doesn't take any interim parents. This doesn't make any sense at all, it should either pick only the full name or any parent name. Attached patch fixes form_get_error() to also check for "interim parents".