Establish the pattern for pipeline_api’s to surface validation errors in a web API response

[natygyoon]

This could be done in several different ways as follows:

• pipeline_api just directly raises a fastapi.HTTPException
• unstructured-api-tools catches ValueError’s and re-raises as fastapi.HTTPException
• an unstructured Exception class is defined in Unstructured , and unstructured-api-tools appropriately handles that