MCKIN-29542 Handling error gracefully if user is not enrolled

[ahmed-zubair12]

Handling error and returning None if user is not enrolled in the course.

[ahmed-zubair12]

@xitij2000 I was getting errors on Python 2.7 and 3.5 builds which were due to f-string use in latest version of edx-rest-api-client. f-strings are only supported in Python 3.6 and above. So i pinned version of edx-rest-api-client in setup.py. This resolved error in Python 3.5 build but getting a new ContextualVersionConflict error in Python 2.7 build. Any ideas on how to resolve it? Also this PR needs to be reviewed by open-craft team.

[xitij2000]

@ahmed-zubair12 Do we still need support for Python 2.7? We can probably just drop it at this point and git rid of this issue.

[mudassir-hafeez]

@xitij2000 I don't think, we need support for Python 2.7 as we are on juniper now on stage. We will be taking this as juniper.
@naeem91 wanted you to be in loop on this.

[xitij2000]

@mudassir-hafeez OK. In that case, you can just edit the circleci config to stop testing on python 2.

[mudassir-hafeez]

We can close this Pr in reference of #44