Refactor error handling in DocumentService and consumers

Refactored `DocumentService.GetDocumentAsync` to throw
`HttpRequestException` on failure instead of returning a tuple.
Updated all consumers to handle exceptions, improving error
handling consistency across the application.

Enhanced error messages for better user feedback and added
logging for improved traceability. Updated `EnvelopeReceiverPage`,
`EnvelopeReceiverPage_DxReportViewer`, and `ReportViewer` to
use the new exception-based API and handle errors gracefully.

This change simplifies the API, improves maintainability, and
makes the application more robust and user-friendly.

EnvelopeGenerator.ReceiverUI/Pages/EnvelopeReceiverPage.razor

@@ -563,13 +563,13 @@ const int MaxThumbnailWidth = 400;
         }
 
         try {
-            var (pdfBytes, statusCode) = await DocumentService.GetDocumentAsync(EnvelopeKey);
+            var pdfBytes = await DocumentService.GetDocumentAsync(EnvelopeKey);
 
             if (pdfBytes is { Length: > 0 }) {
                 var base64 = Convert.ToBase64String(pdfBytes);
                 _pdfDataUrl = $"data:application/pdf;base64,{base64}";
             } else {
-                _errorMessage = $"Dokument konnte nicht geladen werden. HTTP Status: {statusCode}";
+                _errorMessage = "Dokument konnte nicht geladen werden: Keine Daten empfangen.";
             }
 
             var signatures = await SignatureService.GetAsync(EnvelopeKey);
@@ -608,8 +608,12 @@ const int MaxThumbnailWidth = 400;
                 _popupValidationMessage = null;
             }
 
+        } catch (HttpRequestException ex) {
+            _errorMessage = $"Dokument konnte nicht geladen werden: {ex.Message}";
+            logger.LogError(ex, "Failed to load document for envelope {EnvelopeKey}", EnvelopeKey);
         } catch (Exception ex) {
             _errorMessage = $"Fehler: {ex.Message}";
+            logger.LogError(ex, "Unexpected error during initialization for envelope {EnvelopeKey}", EnvelopeKey);
         }
 
         _isLoading = false;

EnvelopeGenerator.ReceiverUI/Pages/EnvelopeReceiverPage_DxReportViewer.razor

@@ -25,11 +25,15 @@
 
     protected override async Task OnInitializedAsync() {
         if (!AppOptions.Value.UsePredefinedReports) {
-            var (pdfBytes, _) = await DocumentService.GetDocumentAsync(EnvelopeKey);
+            try {
+                var pdfBytes = await DocumentService.GetDocumentAsync(EnvelopeKey);
                 if (pdfBytes is { Length: > 0 })
                     BasePdfBytes = pdfBytes;
                 else
                     throw new InvalidOperationException($"No PDF bytes found for EnvelopeKey: {EnvelopeKey}");
+            } catch (HttpRequestException ex) {
+                throw new InvalidOperationException($"Failed to load document for EnvelopeKey: {EnvelopeKey}", ex);
+            }
         }
 
         Report = CreateReport();

EnvelopeGenerator.ReceiverUI/Pages/Example/ReportViewer.razor

@@ -346,9 +346,14 @@ Shown="OnPopupShownAsync">
         _envelopeReceiver = await EnvelopeReceiverService.GetAsync(EnvelopeKey ?? "fake");
 
         if (!AppOptions.Value.UsePredefinedReports && !string.IsNullOrWhiteSpace(EnvelopeKey)) {
-            var (pdfBytes, _) = await DocumentService.GetDocumentAsync(EnvelopeKey);
+            try {
+                var pdfBytes = await DocumentService.GetDocumentAsync(EnvelopeKey);
                 if (pdfBytes is { Length: > 0 })
                     _basePdfBytes = pdfBytes;
+            } catch (HttpRequestException ex) {
+                // Log error but continue - will use predefined report instead
+                Console.WriteLine($"Failed to load document: {ex.Message}");
+            }
         }
 
         var initialReport = BuildFreshBaseReport();

EnvelopeGenerator.ReceiverUI/Services/DocumentService.cs

@@ -11,17 +11,25 @@ public class DocumentService(HttpClient http, IOptions<ApiOptions> apiOptions)
 
     /// <summary>
     /// Fetches the PDF bytes for the given envelope key from the API.
-    /// Returns null bytes with the HTTP status code on failure.
+    /// Throws HttpRequestException on failure with appropriate status code.
     /// </summary>
-    public async Task<(byte[]? Bytes, HttpStatusCode StatusCode)> GetDocumentAsync(string envelopeKey, CancellationToken cancel = default)
+    /// <exception cref="HttpRequestException">Thrown when the API request fails.</exception>
+    public async Task<byte[]?> GetDocumentAsync(string envelopeKey, CancellationToken cancel = default)
     {
         var response = await http.GetAsync($"{_api.BaseUrl}/api/Document/{Uri.EscapeDataString(envelopeKey)}", cancel);
 
         if (!response.IsSuccessStatusCode)
-            return (null, response.StatusCode);
+        {
+            var statusCode = (int)response.StatusCode;
+            var reasonPhrase = response.ReasonPhrase ?? "Unknown error";
+            throw new HttpRequestException(
+                $"Failed to load document. Status: {statusCode} ({reasonPhrase})",
+                null,
+                response.StatusCode);
+        }
 
         var bytes = await response.Content.ReadAsByteArrayAsync(cancel);
-        return (bytes, response.StatusCode);
+        return bytes;
     }
 }