Exception hierarchy: open issues

Hi internals!

There are a number of open issues with regard to the exception hierarchy
for recently introduced exception, for which I would like to open a new
thread:

• Naming and classiness of BaseException. There's an RFC to change this to
  a Throwable interface: https://wiki.php.net/rfc/throwable - If we want to
  move forward with that, it needs a patch, as turning it into an interface
  likely has non-trivial consequences that need to be addressed in the RFC.

• Expectations introduced an AssertionException. Currently it extends
  Exception. Should it extend BaseException instead, as you usually should
  not catch assertion exceptions? This was discussed in the RFC thread, but
  the decision was postponed until engine exceptions land, which has happened
  by now.

• The scalar type hints implementation added a TypeException extends
  EngineException. We didn't discuss this on-list, so I'd like to bring this
  up in case anyone disagrees. Imho TypeException may not be best name for
  it, as it's also thrown for non-type related error conditions, like
  mismatched argument count.

Thanks,
Nikita