Fix usage of message attr in Exception object

[matux]

In Python 3, the message attribute has been removed from Exception and we're expected to obtain its string representation by invoking str on it, eg. str(exc).

In this case, since what's being captured is an AssertionError, message becomes a list where the second element contains the message passed to assert.

We can use args to access this which works on both Pythons.

I've also added tests triggering this assertion condition by passing an invalid sourcemap.

---

Tests ran in Python 2.7

❯ pytest tests
============================ test session starts ============================
platform darwin -- Python 2.7.18, pytest-4.6.11, py-1.11.0, pluggy-0.13.1
rootdir: /Users/matux/code/rollbar/python-sourcemap
collected 17 items

tests/test_init.py ...                                                 [ 17%]
tests/test_integration.py ....                                         [ 41%]
tests/test_objects.py ..........                                       [100%]

============================ 17 passed in 0.12 seconds ======================

Tests ran in Python 3.10

❯ pytest tests
============================ test session starts ============================
platform darwin -- Python 3.10.14, pytest-8.3.3, pluggy-1.5.0
rootdir: /Users/matux/code/rollbar/python-sourcemap
collected 17 items

tests/test_init.py ...                                                 [ 17%]
tests/test_integration.py ....                                         [ 41%]
tests/test_objects.py ..........                                       [100%]

============================ 17 passed in 0.07s =============================