apport_python_hook: FileNotFoundError if cwd was deleted

Impact
------

The Apport Python hook should be transparent to the user (which is normally the case). Bugs and crashes inside the hook lead to an exception inside the exception which clutters the output and confuse the user. User/Developer need to understand this Python hook and look into the encapsulated stack trace only.

Test Case
---------

Run following shell script:

```sh
echo "raise ValueError()" > /tmp/failure.py
mkdir /tmp/foo
cd /tmp/foo/
rmdir /tmp/foo/
python3 ../failure.py
```

It should only print following stacktrace:

```
Traceback (most recent call last):
  File "../failure.py", line 1, in <module>
    raise ValueError()
ValueError
```

Regression Potential
--------------------

There might be a regression that causes capturing Python exception fail. Then reporting them as bugs to Launchpad or the error tracker would not be possible.

Original Description
--------------------

The script should only show a ValueError, but apport_python_hook.py crashes:

```
Traceback (most recent call last):
  File "../failure.py", line 1, in <module>
    raise ValueError()
ValueError
Error in sys.excepthook:
Traceback (most recent call last):
  File "<frozen importlib._bootstrap_external>", line 1372, in _path_importer_cache
KeyError: '..'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 55, in apport_excepthook
    import apt_pkg
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1002, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 945, in _find_spec
  File "<frozen importlib._bootstrap_external>", line 1439, in find_spec
  File "<frozen importlib._bootstrap_external>", line 1408, in _get_spec
  File "<frozen importlib._bootstrap_external>", line 1374, in _path_importer_cache
  File "<frozen importlib._bootstrap_external>", line 1350, in _path_hooks
  File "<frozen importlib._bootstrap_external>", line 1632, in path_hook_for_FileFinder
  File "<frozen importlib._bootstrap_external>", line 1505, in __init__
FileNotFoundError: [Errno 2] No such file or directory

Original exception was:
Traceback (most recent call last):
  File "../failure.py", line 1, in <module>
    raise ValueError()
ValueError
```