Show stack-trace from panic in weave-npc

Rather than exiting silently, fetch the stack trace and print that
for the user to understand what happened.

Exit via existing fatal-error code.

prog/weave-npc/main.go

@@ -2,8 +2,10 @@ package main
 
 import (
 	"context"
+	"fmt"
 	"os"
 	"os/signal"
+	goruntime "runtime"
 	"syscall"
 
 	"github.com/coreos/go-iptables/iptables"
@@ -218,7 +220,9 @@ func hCtx() context.Context {
 
 func stopOnPanicRecover() {
 	if r := recover(); r != nil {
-		os.Exit(1)
+		buf := make([]byte, 1<<20)
+		stacklen := goruntime.Stack(buf, false)
+		handleFatal(fmt.Errorf("panic: %v \n%s", r, buf[:stacklen]))
 	}
 }