PyInterpreter: improve error messaging and traceback

The error message reporting and traceback is corrected and improved.
The full error report is displayed in the GUI and sent to stderr.

Example (stderr):
```
---PyInterpreter: Error:
PyInterpreter:BornAgain: Failed executing Python function 'get_sample'
------------------------------
* Python exception:
No module named 'noneexistantpackage'

Traceback (most recent call last):

  File "<string>", line 6, in get_sample

ModuleNotFoundError: No module named 'noneexistantpackage'

------------------------------
* Runtime environment:
------------------------------
```

Resolves issue #1063