4336: Return 400 error on invalid JSON sent to the datastore API

[janette]

Fixes #4336

Describe your changes

Better response for invalid JSON

QA Steps

• [ ] Run the following POST

  
  POST https://demo.getdkan.org/api/1/datastore/query/cedcd327-4e5d-43f9-8eb1-c11850fa7c55/0 HTTP/1.1

{ "conditions": [ }

- [ ] Confirm you get a 400 error and "Invalid JSON" message

## Checklist before requesting review

_If any of these are left unchecked, please provide an explanation_

- [ ] I have updated or added tests to cover my code
- [ ] I have updated or added documentation