SSE connection disconnects on error

[elliette]

If the Dart Debug Extension stream listening to SSE/WS events receives an error, we disconnect the debug session: https://github.com/dart-lang/webdev/blob/master/dwds/debug_extension/web/background.dart#L569-L577

We should investigate if we are closing the connection too aggressively, and if it would make sense to be more resilient to errors in the stream.

[elliette]

Some ideas:

Instead of immediately disconnecting, we should:

• Send an event to DWDS with the error message
• DWDS should emit an exception event to track the details
• Only after x-number of consecutive error events should we disconnect (eg, 3 error events in a row)