Yosemite + Itunes 12 + ios 8 = iTunes Unexpected Error - Quit
iTunes 12 is the problem, so we need to revert back to 11 to perform update
1) Copy the iTunes 12 library file -/home/youruser/Music/iTunes/iTunes Library.itl to Previous iTunes Libraries in the same diretory and rename to iTunes Library 12.itl
2) Copy the latest .itl file from Previous iTunes Libraries to /home/youruser/Music/iTunes/iTunes Library.itl (my latest was iTunes Library 2012-12-01 1.itl)
3) Uninstall itunes 12. I used appzapper. Download, install, goto preferences, untick 'Protect Apple Applications' then drag itunes from the Applications fold into appzapper. Zap itunes 12.
3) Install itunes 11 from Apple site
4) Restore your phone to ios 8 using the iOS 8 file - Hold option key and press 'Restore iphone' in itunes.
5) Let itunes update your phone to ios 8
6) Do not restore or anything else. Disconnect phone. Quit itunes and install itunes 12 (from Software Update in System Preferences)
7) Now copy back the /home/youruser/Music/iTunes/Previous iTunes Libraries/iTunes Library 12.itl to /home/youruser/Music/iTunes/iTunes Library.itl
8) Open itunes 12 and restore from backup. Alternatively you can restore from icloud on the phone.