apport_python_hook: Crashes if os.getcwd() throws FileNotFoundError

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
---------

1. Create /tmp/getcwd.py with following content:
```
#!/usr/bin/python3

import os
import tempfile

tempdir = tempfile.mkdtemp()
os.chdir(tempdir)
os.rmdir(tempdir)
os.getcwd()
```
2. Make it executable and execute it.

It should produce following output:

```
Traceback (most recent call last):
  File "/tmp/getcwd.py", line 9, in <module>
    os.getcwd()
FileNotFoundError: [Errno 2] No such file or directory
```

instead of:

```
$ /tmp/getcwd.py
Traceback (most recent call last):
  File "/tmp/getcwd.py", line 9, in <module>
    os.getcwd()
FileNotFoundError: [Errno 2] No such file or directory
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 76, in apport_excepthook
    binary = os.path.realpath(os.path.join(os.getcwd(), sys.argv[0]))
FileNotFoundError: [Errno 2] No such file or directory

Original exception was:
Traceback (most recent call last):
  File "/tmp/getcwd.py", line 9, in <module>
    os.getcwd()
FileNotFoundError: [Errno 2] No such file or directory
```

Another related issue: bug #1979637

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.