Startup notification (#250)

Stop the caddy message and start our own init notifications.
Log the version of CoreDNS when starting up.
Fix all middleware's setup functions so that return the error prefixed
with *which* middleware was failing; leads to better debuggable errors
when starting up.

middleware/file/setup.go

@@ -21,7 +21,7 @@ func init() {
 func setup(c *caddy.Controller) error {
 	zones, err := fileParse(c)
 	if err != nil {
-		return err
+		return middleware.Error("file", err)
 	}
 
 	// Add startup functions to notify the master(s).