Install failed due to loss of power and Yosemite will now not go past login screen. Help!

Question

I recently tried to install OS X Yosemite.  unfortunately during the install my laptop was unplugged and the installation failed mid way through.  Each time I start my mac I get to login screen and after logging in get an error message and told to restart and try again - which does not resolve the issue.  Guest login do work to the safari screen.  Any help hugely appreciated. 
The error message reads:
You need to restart your computer.  Hold down the power button until it turns off, then Press the power button again.
Behind system code, including:
panic(CPU 2 caller 0xffffff 80005418bf):
"process 1 exec of /sbin/launchd failed, errno 45"@/sourcecache/xnu/xnu-1699.32.7/bsd/kern/kern_exec.c:3546
Debugger called: <panic>
Backtrace (CPU 2), Frame: Return Address
0Xffffff80e8423e00 : 0xffffff8000220792
7 more lInes like the one above
BSD process name corresponding to current thread: init
MAc OS version:
not yet set
THen kernel version data. 

Answer 1

Try booting into the Safe Mode using your normal account.  Disconnect all peripherals except those needed for the test. Shut down the computer and then power it back up after waiting 10 seconds. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear and again when you log in. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. When you reboot normally, the initial reboot may be slower than normal. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart by using the application un-installer. For each disable/delete, you will need to restart if you don’t do them all at once.
Safe Mode - About
Safe Mode - Yosemite