If the callback raised an exception but did not set curexc_traceback,
the trace function was called with PyTrace_RETURN.  That is, the trace
function was called with an exception set.  The main loop detected the
exception when the trace function returned; it complained and disabled
tracing.

Fix the logic error so that PyTrace_RETURN only occurs if the callback
returned normally.

The trace function must be called for exceptions, too.  So we had
to add new functionality to call with PyTrace_EXCEPTION.  (Leads to a
rather ugly ifdef / else block that contains only a '}'.)

Reverse the logic and name of NOFIX_TRACE to FIX_TRACE.

Joint work with Fred.