Add proper exception handling

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

Start with evalai/submissions.py and inspect each place that fetches entries from HTTP responses. Identify the response keys that may be absent and determine how the existing command flow should handle those cases. Done means missing keys are handled gracefully without an exception escaping unexpectedly.

Written by the indexing model from the issue text.