chore(api/diff): improve error message when json parsing objects

[iainsproat]

Description & motivation

The fact that the objects parameter of the diff upload and download is meant to be JSON stringified representation of a list is often missed by developers (Speckle and non-Speckle, it's really not an obvious API design choice). It is currently difficult to debug because the error message is not helpful.

This PR wraps the JSON parsing error and provides additional context to help make the issue clearer and the problem easier to resolve.

It also ensures the correct status code is returned to the user.

Changes:

To-do before merge:

Screenshots:

Validation of changes:

(NB, the stack trace is only displayed because this is running in development mode)

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