Win32: surface the real error when opening a URL fails

Source/UrlRequest_Win32.cpp

@@ -38,9 +38,15 @@ namespace UrlLib
                 return LoadHttpAsync();
             }
         }
-        catch (winrt::hresult_error)
+        catch (const winrt::hresult_error& error)
         {
-            // Catch WinRT exceptions, but retain the default status code of 0 to indicate a client side error.
+            // Retain the default status code of 0 to indicate a client-side (transport)
+            // error, but record the WinRT failure via SetError -- consistent with the
+            // curl/NSURL backends -- so it is diagnosable through
+            // ErrorString()/ErrorSymbol()/ErrorCode() instead of being silently discarded.
+            std::ostringstream symbol;
+            symbol << "0x" << std::hex << std::uppercase << static_cast<uint32_t>(error.code());
+            SetError("winrt", symbol.str(), static_cast<int32_t>(error.code()), winrt::to_string(error.message()));
             return arcana::task_from_result<std::exception_ptr>();
         }
     }

Source/UrlRequest_Windows_Shared.h

@@ -1,5 +1,6 @@
 #include "UrlRequest_Base.h"
 
+#include <sstream>
 #include <Unknwn.h>
 #include <PathCch.h>
 #include <arcana/threading/task_conversions.h>
@@ -53,7 +54,21 @@ namespace UrlLib
 #endif
 
             m_method = method;
-            m_uri = Foundation::Uri{winrt::to_hstring(url)};
+            try
+            {
+                m_uri = Foundation::Uri{winrt::to_hstring(url)};
+            }
+            catch (const winrt::hresult_error& error)
+            {
+                // Foundation::Uri rejects some inputs (e.g. malformed or unsupported URLs)
+                // by throwing a WinRT hresult_error, which is not a std::exception. Rethrow
+                // as std::runtime_error so callers (e.g. the XMLHttpRequest polyfill) surface
+                // the real reason instead of a generic "Unknown error opening URL".
+                std::ostringstream message;
+                message << "Unable to open URL '" << url << "': " << winrt::to_string(error.message())
+                        << " (HRESULT 0x" << std::hex << std::uppercase << static_cast<uint32_t>(error.code()) << ")";
+                throw std::runtime_error{message.str()};
+            }
         }
 
         arcana::task<void, std::exception_ptr> SendAsync();