When looking at stackdriver make sure to enable all logs at the filter or use this advanced filter:
resource.type="cloud_function"
resource.labels.function_name="<function_name>"
resource.labels.region="<function_region>"
Uncaught exceptions produced by your function will appear in Stackdriver Error Reporting, but if they don't try adding a catch clause and manually write it to stdout
If for some reason you find an error that is not being logged to stackdriver you can reach out to GCP support so they can check if additional logging is available at the particular function termination occurrence.