error messages in shim6

[marcelo bagnulo braun <marcelo@it.uc3m.es>]

Iljitsch has brought up the following issue in his review:

> More in general, most error conditions are handled by silently 
> dropping packets, however, which is a very bad idea because that way, 
> there is no difference between an error and lost messages.
>  So in some cases, a host may continue to resend the offending packet 
> because it doesn't know something went wrong. The main problem with 
> this approach is that you can't debug it from one end: you need to see 
> what happens on both ends to determine why something doesn't work.
>
> Silently dropping packets because of errors is the right approach for 
> security reasons in some cases, but I don't think this applies here. A 
> short error message with an error code and optionally a human-readable 
> message would be much better. As long as these error packets are 
> smaller than the packets that trigger them, there should be little or 
> no security impact, especially considering that we're prepared to talk 
> shim with the correspondent in question to begin with.

his proposal as you can see is to include a shim6 error message to 
handle this situations... any opinions?