memory leak with Python exceptions

[sgillies@…]

Believe it is when we are asserting that map draw should raise an Error.

[sean@lenny python]$ valgrind --tool=memcheck
--suppressions=/home/sean/projects/valgrind-python.supp --num-callers=10
--leak-check=yes python tests/cases/maptest.py

...
==5709==
==5709== 555 bytes in 5 blocks are definitely lost in loss record 66 of 84
==5709==    at 0x1B9054EA: realloc (vg_replace_malloc.c:197)
==5709==    by 0x1C30E57E: msGetErrorString (maperror.c:233)
==5709==    by 0x1C2B518D: _raise_ms_exception (mapscript_wrap.c:846)
==5709==    by 0x1C2D66EE: _wrap_mapObj_draw (mapscript_wrap.c:17266)
==5709==    by 0x80EA40C: PyCFunction_Call (in /usr/bin/python)
==5709==    by 0x805B3B6: PyObject_Call (in /usr/bin/python)
==5709==    by 0x80AB92A: (within /usr/bin/python)
==5709==    by 0x80A93C7: (within /usr/bin/python)
==5709==    by 0x80A9B5D: PyEval_EvalCodeEx (in /usr/bin/python)
==5709==    by 0x80E9CA9: (within /usr/bin/python)
==5709==

[sgillies@…]

We were leaking the error message. Fixed.