What does this implement/fix? Explain your changes.

This introduces a more helpful error when a field in the Schema references a GraphQL Type that has not been registered to the Schema.

Does this close any currently open issues?

closes #3034

Any other comments?

When registering a field that references a Type that doesn't exist:

<?php
add_action( 'graphql_register_types', function() {
  register_graphql_field( 'User', 'fakeField', [
	  'type' => 'NonExistingType'
  ]);
} );

Then querying for the field like so:

{
  users {
    nodes {
      id
      fakeField
    }
  }
}

Before

A vague error was returned with limited information to quickly identify the issue.

After

A clearer Error is returned