bug(ui): connection tree shows generic 'Error' instead of the actual failure message

[ZhuchkaTriplesix]

connections_panel_postgres_connection.dart stores _error = e.toString() but renders only the static text 'Error'.

Impact: Users cannot diagnose why a connection fails to expand in the sidebar.

Expected: Show the actual error message (truncated with tooltip, or expandable).

Acceptance criteria:

• Replace 'Error' with the stored error text, constrained by max lines and tooltip.
• Add widget test verifying the error message is visible.