Allow exit_show_tool_tip from commands with HTML output

The HTML output view will not open before data has been written to stdout. By writing the tool tip to stderr we avoid showing the HTML view (assuming data hasn’t already been written).

This is useful for commands that may fail (or succeed) and wish to convey that with a simple tool tip (previously this required setting the command’s output to something other than New Window / HTML and using exit_show_html, but this made the command run non-detached / blocking).