Don't error if we're never going to use it.

event.go

@@ -1,7 +1,5 @@
 package discordgo
 
-import "fmt"
-
 // EventHandler is an interface for Discord events.
 type EventHandler interface {
 	// Type returns the type of event this handler belongs to.
@@ -45,12 +43,15 @@ var registeredInterfaceProviders = map[string]EventInterfaceProvider{}
 
 // registerInterfaceProvider registers a provider so that DiscordGo can
 // access it's New() method.
-func registerInterfaceProvider(eh EventInterfaceProvider) error {
+func registerInterfaceProvider(eh EventInterfaceProvider) {
 	if _, ok := registeredInterfaceProviders[eh.Type()]; ok {
-		return fmt.Errorf("event %s already registered", eh.Type())
+		return
+		// XXX:
+		// if we should error here, we need to do something with it.
+		// fmt.Errorf("event %s already registered", eh.Type())
 	}
 	registeredInterfaceProviders[eh.Type()] = eh
-	return nil
+	return
 }
 
 // eventHandlerInstance is a wrapper around an event handler, as functions