[Request] Reject PNGs with transparency

[khavishbhundoo]

Since JPEG doesn't suppose transparency , i think it would be more appropriate to output an error like "This image is transparent , it cannot be converted to JPEG".Then we add a flag like --force-convert for people that want to keep the overlaid on black background before encoding behavior ?

[xusai2014]

advise to tell user that the transparent is not supported.