Add handler for context canceled errors

Users of the daemon have reported seeing intermittent errors from the Golang reverse proxy in the daemon due to. They can appear like this:

```
2021-09-25T00:22:54.813+02:00
2021/09/24 22:22:54 http: proxy error: context canceled
2021-09-25T00:22:54.888+02:00
2021/09/24 22:22:54 http: proxy error: context canceled
2021-09-25T00:22:59.888+02:00
2021/09/24 22:22:59 http: proxy error: context canceled
2021-09-25T00:23:01.008+02:00
2021-09-24T22:23:01Z [Info] Successfully sent batch of 9 segments (0.016 seconds)
```

These are from intermittent unexpected severed client connections, which are immediately reconnected upon the next proxy request. They can be safely silenced or logged as a more clear error, which is what other GoLang projects have done, e.g. https://github.com/aerokube/selenoid/pull/631

Start by locating the Go reverse-proxy error logging that emits “http: proxy error: context canceled,” then review the linked Selenoid change for comparison. Done means intermittent severed-client errors are either safely silenced or logged with clearer wording without hiding other proxy errors.

Written by the indexing model from the issue text.