Failed startup was not flagged (#118)

The error propagation from srv.ListenAndServe did not work as intended,
simplified it a bit and added a test for it.

test/fail_start_test.go

@@ -0,0 +1,21 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/miekg/coredns/core"
+)
+
+// Bind to low port should fail.
+func TestFailStartServer(t *testing.T) {
+	corefile := `.:53 {
+	chaos CoreDNS-001 miek@miek.nl
+}
+`
+	srv, _ := core.TestServer(t, corefile)
+	err := srv.ListenAndServe()
+	if err == nil {
+		srv.Stop()
+		t.Fatalf("Low port startup should fail")
+	}
+}