Fix keyframe undo that would cause crash or double undo in some cases (issue #1557)

[aeroc7]

Add boolean return value on backup function so that we can check if the backup was actually successful.

Fixes #1557

[MrStevns]

Hi @aeroc7

Thanks for your contribution! The code looks good to me, so let's just get it in.
There's a larger rework of undo/redo in the works which was started by me, but until that gets done, I think it's a good idea to fix the bugs we can with the current one we have.

Merging 👍

[MrStevns]

👍

[scribblemaniac]

Thanks for the contribution and welcome!