Disable unexpected compilation exceptions caching

[anetaporebska]

Since expression compilation can sometimes throw an unexpected java.lang.NullPointerException: Cannot read the array length because "a" is null due to I/O errors, it would be helpful to have the option to disable such error caching. Otherwise, the cached error is returned every time, paralyzing normal scex usage.

[ddworak]

Side questions:

1. Does it ever make sense to cache errors? I assume it does when they come from actual syntax issues, but maybe we could target those directly then.
2. Are there other errors than NPEs (maybe I/O exceptions?) that should be filtered out before caching?

[anetaporebska]

@ddworak

1. I assume yes, if something goes wrong every time there is no need to recalculate it multiple times.
2. Probably also yes but I don't know any other specific case that breaks because of random errors (such as VM I/O errors)

[freddie713]

Code LGTM, but I would love some unit tests for this new option.

[anetaporebska]

I've decided to extend the solution of disabling caching to all unexpected exceptions (not only NPE) that occur during compilation. This should minimize the probability that any other unexpected exception will block the cache infinitely.

@freddie713 please check the code one more time, now it was easier to unit test it, as I no longer had to rely on the filesystem.

[ddworak]

LGTM

[anetaporebska]

PR summary:

• default CachingScexCompiler behavior has changed - it now caches only expected compilation exceptions (and successes ofc) to avoid caching for example a random NPE thrown during a machine I/O error
• new ScexSettings flag was added - cacheUnexpectedCompilationExceptions - if you rely on such unexpected exceptions you can revert the Scex behavior to the previous one by setting it to true