allow valid error message to be displayed on linux (#18997)

when attempting to enable client debugging on blazor-wasm an attempt to
provide a helpful message casues an ¨Unknown OS platform¨ exception

relates to #16366 #12970

src/Components/Blazor/Server/src/MonoDebugProxy/BlazorMonoDebugProxyAppBuilderExtensions.cs

@@ -337,7 +337,7 @@ namespace Microsoft.AspNetCore.Builder
             }
             else
             {
-                throw new InvalidOperationException("Unknown OS platform");
+                return $@"<p>Edge is not current supported on your platform</p>";
             }
         }