getPath in DirectoryTaxonomyReader should throw an exception [LUCENE-9460]

This issue is a spillover from [LUCENE-9450 PR](https://github.com/apache/lucene-solr/pull/1733) and was suggested by @mikemccand

If the `ordinal` is out of bound it indicates that the user called their main `IndexReader` and the `TaxonomyReader` in the wrong order. 

In this case, we should throw an `IllegalArgumentException` to warn the user instead of returning `null`

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Migrated from [LUCENE-9460](https://issues.apache.org/jira/browse/LUCENE-9460) by Gautam Worah (@gautamworah96), updated Aug 31 2021

Locate DirectoryTaxonomyReader.getPath and read how it handles an out-of-bounds ordinal. Reproduce or inspect the invalid-order case described in the issue, then add coverage where appropriate. Done means an out-of-bounds ordinal raises IllegalArgumentException instead of returning null.

Written by the indexing model from the issue text.