message: better warning for broken data source reference
diff: @@ -25,6 +25,12 @@ module.exports = class ContentType {
  this.id = model.id;
  if (model.relationships['data-source']) {
  this.dataSource = dataSources.get(model.relationships['data-source'].data.id);
  + if (!this.dataSource) {
  + throw new Error(`content type "${model.id}" refers to missing data source id "${model.relationships['data-source'].data.id}"`, {
  + status: 400,
  + title: 'Broken field reference'
  + });
  + }
  } else if (defaultDataSource) {
  this.dataSource = dataSources.get(defaultDataSource.data.id);
  } else {
  