Fix #63: display traceback for exceptions in callables

[qwenger]

Simple attempt for #63 . PyErr_WriteUnraisable seems to fit the bill quite well with minimal work.

Test:

>>> import quickjs
>>> def test():
...     1/0
... 
>>> ctx = quickjs.Context()
>>> ctx.add_callable("test", test)
>>> ctx.eval("test()")
Exception ignored in: <function test at 0x7efe29521940>
Traceback (most recent call last):
  File "<stdin>", line 2, in test
ZeroDivisionError: division by zero
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
_quickjs.JSException: InternalError: Python call failed.
    at test (native)
    at <eval> (<input>)

[qwenger]

Closing in favor of #67.