error returns stacktrace string instead of traceback object

[muszbek]

What used to be:
Upon Python errors Erlang receives an opaque traceback object as binary data. It can be decoded to ASCII as readable text, but it has to be done manually and the non-text bytes obstruct readability.

With this modification:
Erlang now receives the text representation of the traceback as readable binary string.
Better for debugging python processes, better for displaying meaningful error messages to users.