Fixed the processing of Test262Errors to prepend function Test262Error() {} instead of translating them to MjsUnitAssertionError instances. The dummy Test262Error constructor can then be used in assert.throws checks that get converted into assertThrows. As part of this, since the dummy constructor takes no arguments, removed the arguments from the construction site as well.
This fixed a problem where if you used e.g. $ERROR() and throw new Test262Error() in your test262 tests, both would be converted to throwing MjsUnitAssertionError instances, making any assertThrows inaccurate since it would match both.