Chatconv agent: output parser exception (#4923)
the output parser form chat conversational agent now raises
`OutputParserException` like the rest.
The `raise OutputParserExeption(...) from e` form also carries through
the original error details on what went wrong.
I added the `ValueError` as a base class to `OutputParserException` to
avoid breaking code that was relying on `ValueError` as a way to catch
exceptions from the agent. So catching ValuError still works. Not sure
if this is a good idea though ?
