removing .todos.json from a synced list will cause crash

[gammons]

synced lists cannot handle when a synced list's .todos.json is removed.

To repro:

1. Sync a list with ultralist.io
2. rm -rf .todos.json
3. Run ultralist init
4. See crash.

Repo initialized.
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x72a670]

goroutine 1 [running]:
github.com/gammons/ultralist/ultralist.(*EventLogger).LoadSyncedLists(0xc420074300)
        /home/grant/code/go/src/github.com/gammons/ultralist/ultralist/event_logger.go:129 +0x1a0
github.com/gammons/ultralist/ultralist.(*App).InitializeRepo(0xc420095050)
        /home/grant/code/go/src/github.com/gammons/ultralist/ultralist/app.go:36 +0xf9
main.routeInput(0x7fffcf7071b3, 0x4, 0x7fffcf7071b3, 0x4)
        /home/grant/code/go/src/github.com/ultralist/ultralist/main.go:58 +0x791
main.main()
        /home/grant/code/go/src/github.com/ultralist/ultralist/main.go:21 +0xcf

LoadSycnedLists should have a grooming step to remove lists that no longer exist.