404 error trying to download Forge
It looks like the auto-generated URL format doesn't work for this specific version of Forge (MC 1.7.10).
Manually navigated to get this working link: https://maven.minecraftforge.net/net/minecraftforge/forge/1.7.10-10.13.4.1614-1.7.10/forge-1.7.10-10.13.4.1614-1.7.10-installer.jar
Auto-generated, non-working link (404 error): https://files.minecraftforge.net/maven/net/minecraftforge/forge/1.7.10-10.13.4.1614/forge-1.7.10-10.13.4.1614-installer.jar
The modpack causing this issue is here.
I was able to successfully install a different modpack with a different version of Forge.
Hmm, that's interesting. Since there may be more versions that do this, I'll have to add in some additional logic to try the alternate URL if the first one fails.
Also, I should add a link to LexManos's Patreon since this code bypasses the adfly links...
Fixed in 7691391