Ooops, my first version introduces a regression: if file open fails, 
the traceback printing was stopped. Here is a new version of my patch 
to support #coding: header in _Py_DisplaySourceLine(). It doesn't 
print the line of file open fails, but continue to display the end of 
the traceback.

But print still stops on PyFile_WriteObject() or PyFile_WriteString(). 
If PyFile fails, I guess that next print will also fails. (it's also 
the current behaviour of PyTraceBack_Print).

Example:
----
Python 3.0rc1+ (py3k:66643M, Sep 27 2008, 17:11:51)
>>> raise Exception('err')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Exception: err
----

The line is not displayed (why? no idea), but the exception 
("Exception: err") is still displayed.