dak: don't ignore SystemExit

Let the python interpreter handle it and actually exit with the right
status, otherwise when a sub-command calls sys.exit(1) we end up exiting
0 instead, which is confusing if not dangerous.
Signed-off-by: NJulien Cristau <jcristau@debian.org>

dak/dak.py

@@ -228,7 +228,7 @@ def main():
             logger.log([msg])
         sys.exit(1)
     except SystemExit:
-        pass
+        raise
     except:
         if logger:
             for line in traceback.format_exc().split('\n')[:-1]: