getRequiredAttrValue(name) {
    const attribute = this.getAttribute(name);
    if (!attribute) {
      throw new shaka.util.Error(
          shaka.util.Error.Severity.CRITICAL,
          shaka.util.Error.Category.MANIFEST,
          shaka.util.Error.Code.HLS_REQUIRED_ATTRIBUTE_MISSING,
          name);
    }

    return attribute.value;
  }
Finds the attribute and returns its value.
Throws an error if attribute was not found.

@param {string} name
@return {string}