Fix for missing cover 403 on low resolution SLR covers

[vt-idiot]

Never had to do something like this in Go, if there's a more elegant way than checking HEAD for 200 vs. 403 in a nested if/else like this do let me know.

Tested on:

• the problem scene in SLR Images are getting 403 error prior to ~4/2020 #1702
• a currently working scene
• a trans scene

[vt-idiot]

Fixes #1702