Major Description
This custom exception class, added in LUCENE-6046, claims to be Serializable since in inherits from Throwable yet if you try to serialize it you'll hit runtime exceptions because its members don't implement Serializable.
We intentionally pushed Java serialization requirements out of Lucene a while back (LUCENE-2908), but maybe for custom exception classes which unfortunately necessarily claim to implement Serializable we need to do something?
We could just mark the members transient here, but that would mean when you unserialize you get null members on the other end, e.g. you would no longer know which RegExp was problematic ...