Error message for mutation generateCustomerToken

[TomashKhamlai]

Preconditions (*)

1. No preconditions.

Steps to reproduce (*)

1. Execute
   query:

mutation generateCustomerToken(
  $email: String!
  $password: String!
) {
  generateCustomerToken(
    email: $email
    password: $password
  ) {
    token
  }
}

variables:

    {
        "email": "",
        "password": ""
    }

Expected result (*)

1. "message": ""username" is required. Enter and try again."
2. "message": ""password" is required. Enter and try again."

Actual result (*)

{
  "errors": [
    {
      "debugMessage": "One or more input exceptions have occurred.",
      "message": "Internal server error",
      "category": "internal",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "generateCustomerToken"
      ]
    }
  ],
  "data": {
    "generateCustomerToken": null
  }
}

Case should be covered with API-functional tests

[naydav]

#636