Fix Error Message while validating Challenge Config

## From #4278

I am following the instructions from https://eval.ai/web/challenge-create and https://evalai.readthedocs.io/en/latest/host_challenge.html#host-challenge-using-github to host a challenge on https://eval.ai/, but I am getting the following error during the Actions in the build step during 'Validate challenge':

Traceback (most recent call last):
File "github/challenge_processing_script.py", line 74, in
response.raise_for_status()
File "/opt/hostedtoolcache/Python/3.7.5/x64/lib/python3.7/site-packages/requests/models.py", line 941, in raise_for_status
raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 401 Client Error: Unauthorized for url: https://eval.ai/api/challenges/challenge/challenge_host_team/3109/validate_challenge_config/

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "github/challenge_processing_script.py", line 81, in
error = response.json()["error"]
KeyError: 'error'
Error: Process completed with exit code 1.

## Fix:
The EvalAI token had expired. You can check if your token is still valid in Profile -> Auth Token and if it is expired, try generating a new one with Refresh Token.

## Deliverable:
- [ ] Fix the error message to make it more informative for the user so that the user can resolve this error.