Add proper exception handling

[AishwaryJain07]

the file :https://github.com/Cloud-CV/evalai-cli/blob/master/evalai/submissions.py.

here we have several places where we try to fetch entries from the HTTP responses. We want to add proper exception handling in case we do not have the keys present in the response

[DevMhrn]

@AishwaryJain07 This seems to be a good issue , I would like to work on this , please assign it to me