I Believe this goes on the help and support forum... Anywho - I couldn't help you with that, sadly, I don't use mac.

 

@TheTexasPlayer Have you attempted to delete and reinstall the launcher entirely? I've had the issue once where it does get stuck updating and deleting/reinstalling can solve the problem.

Moved to Help/Support (Thanks @SirDaniel13 )

 

I personally do not get it, but I would recommend copying all the files you need in the minecraft folder (Under Application Support), like saves resourcepacks and versions, and then open the launcher again, and it should make a new minecraft folder. This should fix it, but if not, google search to see if anyone else is also getting this error

 

I'm on a Mac as well and I got that message randomly (yesterday) however it was downloaded successfully. Your best bet is to probably redownload the launcher like Goblizz suggested.

 

Hmm, that's odd. The other option I could forsee working would be to back up your screenshots and texture packs to a folder somewhere and delete your .minecraft folder entirely. Then, attempt a re-install like @MaxToMinimum suggested.

If this doesn't work, Google might be a better answer as I've never had an issue more severe than a .minecraft reload.