Youtube import improvments

hi, as a newbie, it would be nice to:

write a warning/error message when no API key is available
write a warning/error message when no API key is not good
show the code error to the user
make a easy-to-understand numbering: Capture_d_écran_vidéo_de_14-05-2018_21_56_36

Edited May 14, 2018 by jean-baptiste