Fix connected Write on snet.Conn

[matzf]

Minimal fix for snet.Conn.Write.
Previously, passing nil into WriteTo would automatically pick the
connected remote address, if available. With some of the previous
refactoring (amongst other, in #3571), this is no longer the case. Right
now, Write is simply broken.
Apply the simplest fix, just specify to use the connected address
(base.remote) in Write. Note that the behaviour is still subtly
different than it used to be (e.g. no error if WriteTo is invoked on a
connected socket), but this seems to be the accepted behaviour change
within the refactoring in #3571.

Additionally, add a specific error message for the nil address case
in WriteTo, as "Unable to write to non-SCION address" was misleading.

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This change is 

[karampok]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @karampok and @matzf)

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go/lib/snet/writer.go, line 74 at r1 (raw file):

		if a == nil {
			return 0, common.NewBasicError("Missing remote address", nil)
		} else {

Ideally we would like to have an implementation without else
Consider something like this

switch x.(type) {
	case nil:
		fmt.Println("x is nil") // here v has type interface{}
	case *myType:
		fmt.Println("x is myType") 
	default:
		fmt.Println("type unknown") // here v has type interface{}
	}

[matzf]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @karampok)

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go/lib/snet/writer.go, line 74 at r1 (raw file):

Previously, karampok (Konstantinos) wrote…

Ideally we would like to have an implementation without else
Consider something like this

switch x.(type) {
	case nil:
		fmt.Println("x is nil") // here v has type interface{}
	case *myType:
		fmt.Println("x is myType") 
	default:
		fmt.Println("type unknown") // here v has type interface{}
	}

Good point, thanks. Done.

[karampok]

Reviewed 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved