Small improvements to portable build script (fixes #157)
Small improvements to portable build script (fixes #157)

 - Make notice that prebuild openslides.exe is used instead of
   compiling it from source a bit friendlier so it isn't confused
   with an error

 - Reduce span of try/except to avoid masking errors when we only
   want to gracefully handle the case that no compiler is present

 - Show "success" message after creating the portable zip

FinnStutzenstein/OpenSlides,OpenSlides/OpenSlides,FinnStutzenstein/OpenSlides,ostcar/OpenSlides,boehlke/OpenSlides,emanuelschuetze/OpenSlides,CatoTH/OpenSlides,rolandgeider/OpenSlides,boehlke/OpenSlides,jwinzer/OpenSlides,boehlke/OpenSlides,rolandgeider/OpenSlides,jwinzer/OpenSlides,CatoTH/OpenSlides,FinnStutzenstein/OpenSlides,normanjaeckel/OpenSlides,emanuelschuetze/OpenSlides,rolandgeider/OpenSlides,tsiegleauq/OpenSlides,emanuelschuetze/OpenSlides,jwinzer/OpenSlides,jwinzer/OpenSlides,tsiegleauq/OpenSlides,ostcar/OpenSlides,ostcar/OpenSlides,emanuelschuetze/OpenSlides,OpenSlides/OpenSlides,boehlke/OpenSlides,CatoTH/OpenSlides,normanjaeckel/OpenSlides,jwinzer/OpenSlides,normanjaeckel/OpenSlides,CatoTH/OpenSlides,FinnStutzenstein/OpenSlides,tsiegleauq/OpenSlides,normanjaeckel/OpenSlides