Improve STOMP on_error handling
Improve STOMP on_error handling

- Remove raising of exception in on_error as it's not the right place to
do that and just leaves a mess in the log.
- Make the error message clearer if the certificate is not authorised.
- Increase log level for these messages to ERROR to match severity.

stfc/ssm,apel/ssm,stfc/ssm,tofu-rocketry/ssm,apel/ssm,tofu-rocketry/ssm