Should prompt when Imgur upload failed

[Vdragon]

Currently it seems that Imgur has an upload limit that will be triggered when uploading too many images in a short period, Hackmd should prompt user the problem instead of silently dropping it

The pad that reproduces the issue : SITCON 2017 R0 共筆 - HackMD

[jackycute]

Hi @Vdragon
I think this is nice to have enhancement.