[feature] try: else and finally blocks

[RiedleroD]

• the else block should run if the try block doesn't fail
• the finally block should always run:
  • after catch/else blocks if they exist
  • before anything outside the try-catch-else-finally blocks run
• some impl notes:
  • make sure break/continue/return don't mess with the execution flow
  • anything besides try should be optional, i.e. you don't need to catch the error to have a finally run