netxlite: improve docs, tests, and code quality (#493)

* netxlite: improve docs, tests, and code quality

* better documentation

* more strict testing of dialer (especially make sure we
document the quirk in https://github.com/ooni/probe/issues/1779
and we have tests to guarantee we don't screw up here)

* introduce NewErrWrapper factory for creating errors so we
have confidence we are creating them correctly

Part of https://github.com/ooni/probe/issues/1591

internal/netxlite/certifi_test.go

@@ -6,8 +6,10 @@ import (
 )
 
 func TestPEMCerts(t *testing.T) {
-	pool := x509.NewCertPool()
-	if !pool.AppendCertsFromPEM([]byte(pemcerts)) {
-		t.Fatal("cannot load pemcerts")
-	}
+	t.Run("we can successfully load the bundled certificates", func(t *testing.T) {
+		pool := x509.NewCertPool()
+		if !pool.AppendCertsFromPEM([]byte(pemcerts)) {
+			t.Fatal("cannot load pemcerts")
+		}
+	})
 }