When javaws shows a LaunchErrorDialog the 'Details' button brings up a dialog with 3-4 tabs:
jnlp file
Exception
Wrapped Exception (if there is one)
Console
The console tab is there because frequently this dialog occurs before the console is shown.
In awt toolkit the console tab shows what would be in the console if it were yet shown.
In the FX toolkit, the console tab only says "not supported yet".
It has been that way for many years, we should implement (or at least remove the tab).