exception refactoring reloaded

* Broke all exception tests, but tests not relying on exceptions
  are working.
* Did some refactoring on Evaluator.escape_clauses, making
  Evaluator.find_match_clause and Evaluator.find_exception_clause
  simpler.
* TODO: Evaluator.find_exception_clause/4 could be more concise, but
  handling missing catch/rescue clauses requires manually specifying
  the quoted tuples.