How to return a GraphQL error in graphql-tools addMockFunctionsToSchema?

I'd like to mock an error response in graphql-tools/addMockFunctionsToSchema mock resolver following this pattern:

const mocks = {
  ...,
  Mutation: () => ({
    getToken: (_, { password }) => {
      if (password === 'password') {
        return { value: casual.uuid }
      } else {
        throw new Error('Incorrect email or password')
      }
    }
  })
}

const schema = makeExecutableSchema({`
  type Token { value: ID! }
  type Mutation {
     getToken(email: String!, password: String!): Token
  }
`});

addMockFunctionsToSchema({ schema, mocks});

This works OK and does return a GraphQL error but:

  1. It looks like it's only returning an error because it's an internal server error, throwing on this line.
  2. I'd like to mock an actual GraphQL error response indicating that the user input was invalid