Fix error msg on `save-grade`

[smadbe]

Sébastien reports me that when using save-grade and the public key is not configured in the platforms table, he gets as error:

{
    "success": false,
    "message": "Bad Request",
    "error_text": "Missing answer_token"
}

Actually he had to read the code to understand it was a public key problem.

This error text seems to be misleading. Please fix it (there should be test about that as well?).