Raise more meaningful error message when environment is not running anymore

Upon environment timeout python client will only receive the error message "Environment in wrong status for call to observations()". Might be good to provide more information why the environment is not running anymore (due to timeout etc.)

```
if (!is_running(self)) {
PyErr_SetString(PyExc_RuntimeError,
"Environment in wrong status for call to observations()");
return NULL;
}
```