[Backend] Handle challenge not found exception in submission worker

[Ram81]

Description

This PR handles gracefully handles the exception at https://github.com/Cloud-CV/EvalAI/blob/master/scripts/workers/submission_worker.py#L693. As we now have Start worker feature available feature in Manage Tab whenever a challenge host tries to start a challenge worker before it is approved the above line fails and raises an exception. This has caused our sentry to be flooded with exceptions. Once auto-launch challenge worker before approval feature is done we will modify the query params used in submission worker which will allow us to start workers even when the challenge is not approved. This fix is to reduce messages being sent to sentry

[RishabhJain2018]

@Ram81 Should we close this PR?