Fix rest_command UnboundLocalError in exception handling (#38656)
Fix rest_command UnboundLocalError in exception handling (#38656)

```
2020-08-07 22:38:10 ERROR (MainThread) [homeassistant.components.websocket_api.http.connection.3903193064] local variable 'response' referenced before assignment
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/components/rest_command/__init__.py", line 115, in async_service_handler
    async with getattr(websession, method)(
  File "/usr/local/lib/python3.8/site-packages/aiohttp/client.py", line 1012, in __aenter__
    self._resp = await self._coro
  File "/usr/local/lib/python3.8/site-packages/aiohttp/client.py", line 582, in _request
    break
  File "/usr/local/lib/python3.8/site-packages/aiohttp/helpers.py", line 586, in __exit__
    raise asyncio.TimeoutError from None
asyncio.exceptions.TimeoutError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/components/websocket_api/commands.py", line 125, in handle_call_service
    await hass.services.async_call(
  File "/usr/src/homeassistant/homeassistant/core.py", line 1281, in async_call
    task.result()
  File "/usr/src/homeassistant/homeassistant/core.py", line 1316, in _execute_service
    await handler.func(service_call)
  File "/usr/src/homeassistant/homeassistant/components/rest_command/__init__.py", line 137, in async_service_handler
    _LOGGER.warning("Timeout call %s", response.url, exc_info=1)
UnboundLocalError: local variable 'response' referenced before assignment
```
tboyce1/home-assistant,GenericStudent/home-assistant,Danielhiversen/home-assistant,aronsky/home-assistant,tchellomello/home-assistant,jawilson/home-assistant,GenericStudent/home-assistant,turbokongen/home-assistant,toddeye/home-assistant,w1ll1am23/home-assistant,mezz64/home-assistant,balloob/home-assistant,partofthething/home-assistant,jawilson/home-assistant,sander76/home-assistant,rohitranjan1991/home-assistant,nkgilley/home-assistant,home-assistant/home-assistant,tboyce1/home-assistant,kennedyshead/home-assistant,mezz64/home-assistant,rohitranjan1991/home-assistant,sander76/home-assistant,tboyce1/home-assistant,sdague/home-assistant,w1ll1am23/home-assistant,turbokongen/home-assistant,tboyce1/home-assistant,balloob/home-assistant,adrienbrault/home-assistant,tboyce021/home-assistant,kennedyshead/home-assistant,sdague/home-assistant,FreekingDean/home-assistant,soldag/home-assistant,rohitranjan1991/home-assistant,home-assistant/home-assistant,tchellomello/home-assistant,partofthething/home-assistant,nkgilley/home-assistant,soldag/home-assistant,toddeye/home-assistant,adrienbrault/home-assistant,lukas-hetzenecker/home-assistant,FreekingDean/home-assistant,balloob/home-assistant,tboyce021/home-assistant,lukas-hetzenecker/home-assistant,Danielhiversen/home-assistant,aronsky/home-assistant