A: you are unable to load the game after install as the launcher keeps saying above error..
B: you are on a college network.. or a network with firewall restrictions on P2P downloading..
C: are from the United Kingdom..
then this is the fix for you ...
go to ... http://patchup.info/d3/tfil/
select from the tabs - AMERICAS (NOT EUROPE!!)
download and unzip .tfil file into your diablo3 folder
I was trying this with the europe .tfil file and it didn't work. so tried Americas .tfil and it worked first time
also.. as a side note.. you might want to delete your \battle.net folder within C:\ProgramData.. to let diabo remake folders for a fresh approach, though I found was not necessary.
good luck and hope this helps!!