Fix #106: Display error message while submitting when user doesn't participate in a challenge

[hkmatsumoto]

Fix #106

The corresponding GCI task

Changes:

• Add lines to display an error message when a user submitting to the challenge s/he doesn't participate.

@vkartik97 @pushkalkatara
Could you take a look? Thanks in advance!

[hkmatsumoto]

Brief explanation of the logic of this code lines:
When a user submits to the challenges s/he doesn't participate using cli, these lines in EvalAI will be called and

{"error": "You haven't participated in the challenge"}

is returned with 403 status code.

Thus, to detect the error I check if the returned status code equals to 403, and
if so print the ["error"] of the returned json which is "You haven't participated in the challenge", as you see in the json code above.

[hkmatsumoto]

Screencast:

[yashdusing]

LGTM 👍

[hkmatsumoto]

@yashdusing Thanks! Can I also get my task approved?