Remove a try finally in engine server task.py

This throws a real exception and avoids misleading error:

```python
ERROR local variable 'url' referenced before assignment
Traceback (most recent call last):
  File "/home/openquake/GEM/openquake/oq-engine/openquake/server/tasks.py", line 69, in safely_call
    func(*args)
  File "/home/openquake/GEM/openquake/oq-engine/openquake/server/tasks.py", line 100, in run_calc
    update_calculation(callback_url, status="started", engine_id=calc_id)
  File "/home/openquake/GEM/openquake/oq-engine/openquake/server/tasks.py", line 182, in update_calculation
    url.close()
UnboundLocalError: local variable 'url' referenced before assignment
```