Corrupt Photos Libray when migrating to new Photos.app
Since Photos is so new, it may be tricky to find solutions to issues.  However, applying iPhoto type fixes may help.  

Before beginning and repair, make sure you have a backup of your entire Photos/iPhoto library.  


You can try closing Photos, then open Photos while pressing Command and Option.  This brings up a window for repairing issues.  Select the Repair option to start rebuilding the library.  These instructions can also be found HERE on Apple's website.
Unfortunately, the wonderful app, iPhoto Library Manager doesn't support Photos.app, but they are creating a new app that does, call PowerPhotos!  Hopefully it'll be released soon.  Until then, we must rely on Apples built in repair functions.