Issue #1586: Symtab: Add c.l.Error to the symbol table (#1586).
Issue #1586:
AbstractModelLoader: Change the model loader for exception types
AbstractTransformer: Don't erase to j.l.Error or j.l.Exception when extending and instantiating.
We no longer need the extra parameter about the @Overrides-ness of getType() method.
Issue #1586: AbstractModelLoader: Special case for setting the uncheckedNullType-ness of exception messages -- maybe @FroMage can see a nicer way to achieve this.
Issue #1586: Add a test for the uncheckedNullType-ness of exception messages.
Issue #1586: Add a test and update tests for the non-erasure of exception types in extends clauses and at instantiations.
Issue #1586: Update tests now that j.l.Error and j.l.Exception are not importable.