This should really be added to the FAQ. Alright it works. Here's what you do.
Step 1: Open up the error log and look for the links being downloaded RIGHT before the error occurs
Step 2: Go to http://files.minecraftforge.net/fmllibs/ and download the .jar's that the launcher was supposed to download.
Step 3: Delete all the files in [modpack]/lib to make sure your files remain uncorrupted
Step 4: Move all the libs you downloaded into the lib folder
Step 5: Close and launch, it should be working.
The best explanation I can come up with is your OS or antivirus isn't letting you download the stuff properly or maybe it's some overlooked bug. If this doesn't work for you then I don't know what to tell you. But it should work.