If anything wrong occurs when using the web interface, an empty output is 
presented and no error message is shown.

Change that so the error message appears.

This is useful if the user tried to use %!include or %!csv commands and the 
file (obviously) is not found. Without the error message, she will have no clue 
that these commands are the culprits.

Should not be hard to implement for a PHP developer. Must edit 
extras/txt2tags.php and search for "Compose command line". Check if running the 
command returned an error (catch STDERR) and show it.

Any volunteer?