[io] Clarify the meaning if 'unsolicited response' exception

Bug:#60333
Change-Id: Ife1adc7d50d08810d662cb77ec16fafb1a5fa2b5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/427042
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Brian Quinlan <[email protected]>

Author: brianquinlan

sdk/lib/_http/http_impl.dart

@@ -2201,7 +2201,11 @@ class _HttpClientConnection {
         // We assume the response is not here, until we have send the request.
         if (_nextResponseCompleter == null) {
           throw HttpException(
-            "Unexpected response (unsolicited response without request).",
+            "Unexpected response (unsolicited response without request). This "
+            "can be caused when a malformed request is sent to the server "
+            "(e.g. a GET request containing a body) or if the server is not "
+            "correctly implemented. Future requests to this server, using the "
+            "same HttpClient, are likely to fail.",
             uri: _currentUri,
           );
         }