This was pretty frustrating because the game could always download them before. Oddly enough, the files would need re-downloaded each time I launched the game, which never made sense to me, but whatever. The problem of course comes when the server hosting those files either removes or changes them... which is what happened.
After some searching I found a post on the Minecraft forums with a simple explanation and permanent fix, which I will now be sharing here.
User funjust0 writes:
I've found the solution:Not only will this get your game to work, it will speed load times slightly by not having to download the files each time you launch!
the launcher is looking for scala-library.jar and bcprov-jdk15on-148.jar.
The problem is that the owner(s) of the site http://files.minecraftforge.net have changed this files to stash files
so now they are called scala-library.jar.stash and bcprov-jdk15on-148.jar.stash.
So you'll have to download them here:
(same sites as before, but now with .stash)
put this files into your lib folder and change their names to what you need (remove the .stash at the end)
You will get a message that the extension will change, but you can ignore that.
If you now start the launcher again, It will already find the files and it wont try to download it anymore
So you're ready to play (this worked for me anyway)
In other news, MultiMC5 is in an open beta/testing stage and so far seems to be doing well. MMC5 works for Minecraft 1.6.4 and lower (though you may have to use the fix above for 1.5.2 and older instances). For all the features added to the vanilla launcher, it's still primitive compared to even the beta version of MultiMC.
If you want to give MultiMC5 a try, here's a direct download link. If you have any questions/suggestions/praise/etc. be sure to post it on their forum thread!