better error message for \begin{sparagraph}[name=... outside an smodule

[kohlhase]

I get the error message

! LaTeX Error: Variable \c_stex_module__symbols_prop undefined.

For immediate help type H <return>.
 ...                                              
                                                  
l.34 ...raph}[title=AI Conundrum,name=aiconundrum]
                                                  

! Package stex Error: No symbol aiconundrum found!

Type <return> to continue.
 ...                                              
                                                  
l.39     \begin{sexample}[for=aiconundrum]
                                          
)

! LaTeX Error: File `/Users/kohlhase/localmh/MathHub/mkohlhase/talks/source/AI/slides?ai-summer-winter.en' not found.

and that is not very helpful, especially for sTeX beginners.