Unclear error for wrong username/password

Description

Steps to reproduce:

1. Post an API request to create an access token using non-existent username and/or password:

{{EPCMSHost}}/api/episerver/auth/token

Expected:
Returns response code 401 Unauthorized and a clear error message indicating invalid username/password.

Actual:
Returns response code 400 Bad Request with response body below.

"error": "invalid_grant"

Note: 400 error should appear in the case of invalid/ill-formatted value of request parameters.