[io] HttpClient: add a more descriptive message if the HTTP
authentication challenge is incorrectly formatted.

Bug:https://github.com/dart-lang/sdk/issues/57117
Change-Id: Iceace8ae5c43293f66c2167e2c5ee598e5fecb54
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/402280
Commit-Queue: Brian Quinlan <bquinlan@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>

sdk/lib/_http/http_impl.dart

diff --git a/sdk/lib/_http/http_impl.dart b/sdk/lib/_http/http_impl.dart
index 46cee63..1fe08b6 100644
--- a/sdk/lib/_http/http_impl.dart
+++ b/sdk/lib/_http/http_impl.dart
@@ -845,10 +845,18 @@
 
     List<String> challenge = authChallenge()!;
     assert(challenge.length == 1);
-    _HeaderValue header = _HeaderValue.parse(
-      challenge[0],
-      parameterSeparator: ",",
-    );
+    final _HeaderValue header;
+    try {
+      header = _HeaderValue.parse(challenge[0], parameterSeparator: ",");
+    } on HttpException catch (_, s) {
+      Error.throwWithStackTrace(
+        HttpException(
+          'The authentication challenge sent by the server is '
+          'not correctly formatted.',
+        ),
+        s,
+      );
+    }
     _AuthenticationScheme scheme = _AuthenticationScheme.fromString(
       header.value,
     );