Implement chained exceptions

[jepler]

This should work substantially like Python. Two tests are added.

>>> try:
...     1/0
... except Exception as e:
...     raise RuntimeError("I am sad")
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
RuntimeError: I am sad 

TODO:

• When the outer exception is raised implicitly (e.g., by computing 1/0) rather than by raise the context is not set

[dhalbert]

Thank you!!