Enforcing better error messages in uglify

[avdg]

As proposed in tc39/test262#196 (notified from tc39/test262#671 (comment)), test262 coverage may include well formatted error messages.

I still have to take a look into this one because this could involve in very many additional errors the test suite and would require mocha tests to be updated as well.

The upside is that there is a certain probability that the python test262 tester might not get this update. But since this tool is declared officially deprecated in tc39/test262#647, this still might have an effect later on.

So to reduce impact, it might be better to look at the required changes and take an inventory of them as soon as possible.

[avdg]

There is no need to fix this problem on master, though consistent errors might be welcome.

[avdg]

I might have to switch to https://github.com/bterlson/test262-harness over time as alternative.

[avdg]

Master branch should be fine for now. Now it's just a wait and see game while master merges with harmony and then having to fix harmony as well.

[kzc]

@avdg In your test262 testing do you enable mangle?

[avdg]

https://github.com/avdg/UglifyJSMonitor/blob/master/scripts/ecmaTester.js#L33

[avdg]

Seems harmony is fine after merge. Closing.