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/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);
-            if (pdfBytes is { Length: > 0 })
-                _basePdfBytes = pdfBytes;
+            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();