[smolagents] improve display of handled errors

[axiomofjoy]

smolagents raises errors in some child spans, but handles them in the parent span (e.g., when lexing and parsing generated code with code agents). It is helpful for debugging purposes to have the error details, but the trace tree in the trace details slideover makes it seem as though the exception is unhandled. Ideally, this information would be presented in a way that doesn't seem like an unhandled exception.

Context: #1184 (comment)