Include stacktrace of exception cause in stacktrace

[ewjoachim]

>>> import traceback
>>> try:
...     try:
...         raise ValueError
...     except ValueError as exc:
...         raise IndexError from exc
... except IndexError:
...     import sys
...     exc_type, exc_value, exc_traceback = sys.exc_info()
...     print(traceback.format_tb(exc_traceback))
...
['  File "<python-input-3>", line 5, in <module>\n    raise IndexError from exc\n']
>>> try:
...     try:
...         raise ValueError
...     except ValueError as exc:
...         raise IndexError from exc
... except IndexError:
...     import sys
...     exc_type, exc_value, exc_traceback = sys.exc_info()
...     print(traceback.format_exception(exc_value))
...
['Traceback (most recent call last):\n', '  File "<python-input-5>", line 3, in <module>\n    raise ValueError\n', 'ValueError\n', '\nThe above exception was the direct cause of the following exception:\n\n', 'Traceback (most recent call last):\n', '  File "<python-input-5>", line 5, in <module>\n    raise IndexError from exc\n', 'IndexError\n']

As you can see, if we use traceback.format_exception, we end up having the full exception, including its causes, whereas with format_tb we only have the last exception which can be far less informative.

This PR makes the exception tracebacks more informative.

[SuperDormito]

LGTM

[ewjoachim]

@ybastide I don't understand your last commit ?

[ewjoachim]

HAHA it depends on the python version. They don't all print tracebacks the same way :D