Invalid submitted json is reported as server error

[rimas-kudelis]

API Platform version(s) affected: 2.6.8

Description
When invalid JSON is being submitted to a POST endpoint, a 500 status code is being returned even though invalid input is a user error, not that of the server. A proper status code for this case would be 400, I think.

How to reproduce
Submit intentionally broken JSON body to a POST endpoint, e.g., without a closing brace.

For a quick demo:

1. Open https://demo.sylius.com/api/v2/.
2. Find the group of requests named "Customer".
3. Expand the POST endpoint
4. paste the following body and submit the request:

   {
     "firstName": "John",
     "lastName": "Doe",
     "email": "[email protected]",
     "password": "Password",
     "subscribedToNewsletter": true
   

5. Note that you get a 500 server error. If you add the missing brace, you'll get an expected response code (204 or 422).