Improve the error handling on some worker management API calls, to deal
with closed HTTP connections better.
A new function, `api_impl.handleConnectionClosed()` can now be called when
`errors.Is(err, context.Canceled)`. This will only log at debug level, and
send a `419 I'm a Teapot` response to the client. This response will very
likely never be seen, as the connection was closed. However, in case this
function is called by mistake, this response is unlikely to be accepted
by the HTTP client.
6c2d3d7fc0