If SSE session from browser is getting terminated by proxy, adk web should reopen it

# Current behavior

Currently, if `/run_sse` request is getting terminated by some proxy like nginx, ADK WEB just displays a `TypeError: "network error"` popup window (that can't be easily traced to anything in any way BTW, there is no detailed error in the browser console or anywhere else, that disappears after several seconds and does nothing.

# Expected behavior

ADK should reestablish the SSE request if this happens and there are no persistent connectivity or networking issues. Terminating proxies are a common occurrence.

Also, legitimate network errors should ideally leave some trace, not just show a transient pop-up for a couple of seconds. They should also carry a more descriptive message.