Subclass the built-in `TimeoutError`

[troyraen]

When astroquery.exceptions.TimeoutError is raised, the traceback is misleading because it looks like it's actually python's built-in TimeoutError that is being raised. An example is here: nasa-fornax/fornax-demo-notebooks#437 (comment). @bsipocz suggested in nasa-fornax/fornax-demo-notebooks#592 that astroquery should subclass the built-in error so that try/excepts that catch the built-in would also catch astroquery.exceptions.TimeoutError.