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_DxReportViewer.razor

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