Make it explicit when we are intentionally not checking errors

The linter complains that we're not checking for errors, but on some
cases it's on code paths were it is reasonable to do so (e.g. we're
closing the connection and it's a best-effort write).

This patch adjusts the code to make those cases explicit.

internal/courier/smtp.go

@@ -220,7 +220,7 @@ retry:
 		return a.tr.Errorf("DATA closing %v", err), smtp.IsPermanent(err)
 	}
 
-	c.Quit()
+	_ = c.Quit()
 	a.tr.Debugf("done")
 
 	return nil, false