Better error handling, many fixes all around

gemini/errors_test.go

@@ -0,0 +1,24 @@
+package gemini
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestErrGemini(t *testing.T) {
+	t.Parallel()
+	err := NewErrGeminiStatusCode(50, "50 server error")
+	if !errors.As(err, new(*GeminiError)) {
+		t.Errorf("TestErrGemini fail")
+	}
+}
+
+func TestErrGeminiWrapped(t *testing.T) {
+	t.Parallel()
+	err := NewErrGeminiStatusCode(50, "50 server error")
+	errWrapped := fmt.Errorf("%w wrapped", err)
+	if !errors.As(errWrapped, new(*GeminiError)) {
+		t.Errorf("TestErrGeminiWrapped fail")
+	}
+}