[MIN] Improve participant error message
[MIN] Improve participant error message

- display a less confusing message when trying to apply as a
  participant for a meeting where you already are a participant.
- fixes #543

pferreir/indico,ThiefMaster/indico,pferreir/indico,mvidalgarcia/indico,DirkHoffmann/indico,indico/indico,OmeGak/indico,ThiefMaster/indico,OmeGak/indico,pferreir/indico,mic4ael/indico,indico/indico,OmeGak/indico,DirkHoffmann/indico,mic4ael/indico,OmeGak/indico,mic4ael/indico,mvidalgarcia/indico,indico/indico,pferreir/indico,DirkHoffmann/indico,mic4ael/indico,mvidalgarcia/indico,DirkHoffmann/indico,indico/indico,mvidalgarcia/indico,ThiefMaster/indico,ThiefMaster/indico