error_all_requests hides original exception details

[dkropachev]

error_all_requests() at connection.py:1035 always wraps the error:

new_exc = ConnectionShutdown(str(exc))

This means whether the connection failed due to:

• Expected closure: driver shutting down, graceful server disconnect
• Unexpected I/O failure: ECONNRESET, EPIPE, EBADF mid-operation

...pending requests always receive ConnectionShutdown. The retry policy and Session._handle_error cannot distinguish between these cases.

This matters because:

• Expected closures are safe to retry immediately on another host
• Unexpected failures might indicate a problematic node (worth logging warnings, backing off)

Possible fix direction: Preserve the original exception through error_all_requests(), or introduce error subclasses that carry both the shutdown signal and the underlying cause.