Add ability to try-catch exceptions in MuranoPL

Add language construct to MuranoPL that is semantically equivalent of Python's try-catch-finally-else. Add number of general purposed exception types and define their hierarchy. It must be possible to define new exception types. Exception handling needs to be flexible with ability to catch only desired exception types. Unhandled exceptions must cause execution stack unwind (as in Python, Java and other languages) and can be caught in outer scope