smtpsrv: Don't consider client EOF an error

When the client closes the connection, which is very common, chasquid
logs it as an error ("exiting with error: EOF").

That can be confusing and mislead users, and also makes a lot of
traces be marked as errored, when nothing wrong occurred.

So this patch changes the log to not treat it as an error.
3 jobs for next in 9 minutes and 14 seconds
Status Job ID Name Coverage
  Test
passed #480509405
integration_test

00:05:37

passed #480509406
integration_test_latest

00:05:16

 
  Docker Image
passed #480509407
image_build

00:03:37