fix(server): diff upload and download can handle JSON parsing errors

[iainsproat]

Description & motivation

Previously, a 500 error was returned on invalid user input. This change returns 400 error with helpful error message to guide the user towards a fix.

Error identified via fuzz testing.

Changes:

• test for error case provided
• handling of JSON parsing made more robust and correct error status code returned
• unrelated test, but which relied on this endpoint, amended to expect the updated status code

To-do before merge:

Screenshots:

Validation of changes:

Checklist:

• My pull request follows the guidelines in the Contributing guide?
• My pull request does not duplicate any other open Pull Requests for the same update/change?
• My commits are related to the pull request and do not amend unrelated code or documentation.
• My code follows a similar style to existing code.
• I have added appropriate tests.
• I have updated or added relevant documentation.

References