It should not be possible for the launcher to even try to download Forge's jar, it is packed into the installer.
Try this, go into C:\Users\[Your Username]\AppData\Roaming\.minecraft\libraries\net\minecraftforge\forge\1.10-12.18.0.2000-1.10.0
if there is a forge jar file in there, delete it and rerun the installer,
if there is no forge jar or this folder does not exist, run the installer from the command line and post the output as well as the launcher's log.