If you send a request to the REST API with invalid JSON in body than it will now return a error. This assists developers if they accidentally send invalid JSON and wonder why their data appears to be ignored.
Props rmccue.
Fixes#38547.
git-svn-id: https://develop.svn.wordpress.org/trunk@39109 602fd350-edb4-49c9-b593-d223f7449a82