handleCodeUnitLoadError(codeUnit) {
    var message = codeUnit.error ? String(codeUnit.error) + '\n' :
        `Failed to load '${codeUnit.address}'.\n`;
    message += codeUnit.nameTrace() + this.loader_.nameTrace(codeUnit);

    this.rejectOneAndAll(codeUnit, new Error(message));
  }
This is called when a code unit failed to load.
@param {CodeUnit} codeUnit