Add error handling when sending invoice

Today (2024-06-10) after sending the invoice
we received the following error:

```xml
<?xml version="1.0" encoding="utf-8"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
  <soapenv:Body>
    <ns2:rispostaSdIRiceviFile
      xmlns:ns2="http://www.fatturapa.gov.it/sdi/ws/trasmissione/v1.0/types">
      <IdentificativoSdI>0</IdentificativoSdI>
      <DataOraRicezione>2024-06-10T16:00:00.003+02:00</DataOraRicezione>
      <Errore>EI02</Errore>
    </ns2:rispostaSdIRiceviFile>
  </soapenv:Body>
</soapenv:Envelope>
```

Code "EI02" (SERVIZIO NON DISPONIBILE) means service unavailable.
SdI (Sistema di Interscambio) was not working correctly
but `SendInvoice` function returned error as a nil.

This change fixes that.

Now CLI returns error about service unavailable.

```console
$ go run ./cmd/gobl.fatturapa transmit --ca-cert ./ca-all.pem --cert ./SDI-B85905495.pem --key ./key_client.key --env test invoice.xml
...
sending error: service unavailable
exit status 1
```

sdi/invoice.go

@@ -45,7 +45,7 @@ func SendInvoice(ctx context.Context, invOpts InvoiceOpts, c *Client, cfg Config
 		return nil, err
 	}
 
-	return response, nil
+	return response, getErrorMessageFromResponse(*response)
 }
 
 // SoapRequestToSendInvoice prepares the request content for SOAP to send an invoice
@@ -63,3 +63,38 @@ func SoapRequestToSendInvoice(fileName string, fileBody []byte) string {
 		`</soapenv:Body>` +
 		`</soapenv:Envelope>`
 }
+
+const (
+	// EmptyFileError represents error for empty invoice file
+	EmptyFileError = "EI01" // FILE VUOTO
+
+	// ServiceUnavailableError represents error when service is unavailable
+	ServiceUnavailableError = "EI02" // SERVIZIO NON DISPONIBILE
+
+	// InactiveUserError represents error for inactive user
+	InactiveUserError = "EI03" // UTENTE NON ABILITATO
+
+	// IncorrectFileTypeError represents error for incorrect file type
+	IncorrectFileTypeError = "EI04" // TIPO FILE NON CORRETTO
+)
+
+func getErrorMessageFromResponse(response SendInvoiceResponse) error {
+	respErr := response.Body.Response.Error
+	if respErr == nil {
+		return nil
+	}
+
+	errors := map[string]string{
+		EmptyFileError:          "empty file",
+		ServiceUnavailableError: "service unavailable",
+		InactiveUserError:       "inactive user",
+		IncorrectFileTypeError:  "incorrect file type",
+	}
+
+	errCode := errors[respErr.ErrorCode]
+	if errCode == "" {
+		return fmt.Errorf("unknown error code: %v", respErr)
+	}
+
+	return fmt.Errorf(errCode)
+}

sdi/invoice_test.go

@@ -18,6 +18,66 @@ import (
 )
 
 func TestSendInvoice(t *testing.T) {
+	t.Run("should return error for empty invoice file", func(t *testing.T) {
+		ctx := context.Background()
+		cfg := sdi.DevelopmentSdIConfig
+
+		xop := `
+--MIMEBoundary_000000000000000000000000000000000000000000000000
+Content-Type: application/xop+xml; charset=utf-8; type="text/xml"
+Content-Transfer-Encoding: binary
+Content-ID: <[email protected]>
+
+<?xml version="1.0" encoding="utf-8"?>
+<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
+  <soapenv:Body>
+    <ns2:rispostaSdIRiceviFile
+      xmlns:ns2="http://www.fatturapa.gov.it/sdi/ws/trasmissione/v1.0/types">
+      <IdentificativoSdI>0</IdentificativoSdI>
+      <DataOraRicezione>2024-06-10T17:00:00.400+02:00</DataOraRicezione>
+      <Errore>EI01</Errore>
+    </ns2:rispostaSdIRiceviFile>
+  </soapenv:Body>
+</soapenv:Envelope>
+--MIMEBoundary_000000000000000000000000000000000000000000000000--
+`
+
+		expectedResponse := sdi.ReceiveFileResponse{
+			SdiIdentifier: "0",
+			ReceiptTime:   "2024-06-10T17:00:00.400+02:00",
+			Error:         &sdi.ErrorReceipt{ErrorCode: "EI01"},
+		}
+
+		invOpts := sdi.InvoiceOpts{
+			FileName: "FILENAME.xml",
+			FileBody: []byte(""),
+		}
+
+		client := resty.New()
+
+		// block all HTTP requests
+		httpmock.ActivateNonDefault(client.GetClient())
+
+		header := http.Header{}
+		header.Set("Content-Type", `multipart/related; boundary="MIMEBoundary_000000000000000000000000000000000000000000000000"; type="application/xop+xml"; start="<[email protected]>"; start-info="text/xml"`)
+		header.Set("X-Powered-By", "Servlet/3.0")
+		responder := httpmock.NewStringResponder(200, xop).HeaderAdd(header)
+		httpmock.RegisterResponder("POST", cfg.SOAPReceiveFileEndpoint(), responder)
+
+		c := sdi.NewClient(
+			sdi.WithClient(client),
+			sdi.WithDebugMode(true),
+		)
+
+		response, err := sdi.SendInvoice(ctx, invOpts, c, cfg)
+		require.Error(t, err)
+		require.EqualError(t, err, "empty file")
+
+		assert.Equal(t, expectedResponse, response.Body.Response)
+
+		httpmock.DeactivateAndReset()
+	})
+
 	t.Run("should return receive file response", func(t *testing.T) {
 		ctx := context.Background()
 		cfg := sdi.DevelopmentSdIConfig