Resolve "Cannot add participants to my course"

What does this mr do?

Give a clearer error message to the user when the username provided cannot be found.

Screenshots

before

after

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made.
• Tests were created to test the feature.
• I have updated the documentation accordingly.
• I adhere to the style guide.

Closes #496 (closed)