python: Add exception for wmfs.init()

if wmfs is not running, an exception is called like:
  In [2]: wmfs.init()
  ---------------------------------------------------------------------------
  error                                     Traceback (most recent call last)

  /usr/home/marc/<ipython console> in <module>()

  error: WMFS not running?

Signed-off-by: Marc Lagrange <marc@shebang.sangor>

python/libwmfs.c

@@ -92,9 +92,9 @@ wmfs_init(PyObject *self, PyObject *args)
      /* Else, exit. */
      if(!ret)
      {
-          fprintf(stderr, "WMFS is not running.\n");
           XFree(ret);
-          exit(EXIT_FAILURE);
+          PyErr_SetString(WmfsInitError, "WMFS not running?");
+          return NULL;
      }
      XFree(ret);