Gracefully handle the error when visiting the course resources routes that does not exist

[yash-learner]

From rollbar error

Describe the bug
Use find_by instead of find and gracefully handle the error. find would raise an Active Record Not Found error when a record is not found and would get caught by Rollbar this happens when users try to resource which does not exist, whereas find_by would return nil without raising an error but nil need to handled.

To Reproduce
Steps to reproduce the behavior:

1. Go to course curriculum or show page which does not exist. school.localhost/courses/:id
2. It will raise Active Record Not Found
3. It will get caught by Rollbar.

Expected behavior
Use find_by and gracefully handle the error

[yash-learner]

Team decided to record these 404's in Rollbar as it is expected behaviour, when a record is not found