exchange_rate: don't log full exception trace for timeouts

```
20220222T134125.306163Z |    ERROR | exchange_rate.CoinGecko | failed fx quotes: TimeoutError()
Traceback (most recent call last):
  File "...\electrum\electrum\exchange_rate.py", line 81, in update_safe
    self.quotes = await self.get_rates(ccy)
  File "...\electrum\electrum\exchange_rate.py", line 306, in get_rates
    json = await self.get_json('api.coingecko.com', '/api/v3/exchange_rates')
  File "...\electrum\electrum\exchange_rate.py", line 65, in get_json
    async with session.get(url) as response:
  File "...\Python39\site-packages\aiohttp\client.py", line 1138, in __aenter__
    self._resp = await self._coro
  File "...\Python39\site-packages\aiohttp\client.py", line 634, in _request
    break
  File "...\Python39\site-packages\aiohttp\helpers.py", line 721, in __exit__
    raise asyncio.TimeoutError from None
asyncio.exceptions.TimeoutError
```

electrum/exchange_rate.py

@@ -80,7 +80,7 @@ class ExchangeBase(Logger):
             self.logger.info(f"getting fx quotes for {ccy}")
             self.quotes = await self.get_rates(ccy)
             self.logger.info("received fx quotes")
-        except aiohttp.ClientError as e:
+        except (aiohttp.ClientError, asyncio.TimeoutError) as e:
             self.logger.info(f"failed fx quotes: {repr(e)}")
             self.quotes = {}
         except Exception as e:
@@ -111,7 +111,7 @@ class ExchangeBase(Logger):
             self.logger.info(f"requesting fx history for {ccy}")
             h = await self.request_history(ccy)
             self.logger.info(f"received fx history for {ccy}")
-        except aiohttp.ClientError as e:
+        except (aiohttp.ClientError, asyncio.TimeoutError) as e:
             self.logger.info(f"failed fx history: {repr(e)}")
             return
         except Exception as e: