Fix PL/Python traceback for error in separate file

It assumed that the lineno from the traceback always refers to the
PL/Python function.  If you created a PL/Python function that imports
some code, runs it, and that code raises an exception, PLy_traceback
would get utterly confused.

Now we look at the file name reported with the traceback and only
print the source line if it came from the PL/Python function.

Jan Urbański