When raising an exception cgilib creates a html-error page.
The javasript does not interpret that as a valid response and the client
sees *no* response.

This change writes an error message to the client. Thus there is at
lease some feedback in the ui.

Pattern was changed by kasa but does not match. Hotfix is to revert the
change.

Signed-off-by: Chris Fiege <chris@tinyhost.de>