Needs a way to notify broken sketch folder
## Description
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Provide a detailed description of the behavior you're seeing or the behavior you'd like to see **below** this comment.
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Sometimes, generated folder is broken while using Kactus (though it is not the scope of this issue)
When this happens, There is no notification that the folder is broken when I regenerate the sketch file.
It leads for user to committing further not knowing the folder is broken & making harder to fix as time goes.

## Version
0.3.3

* Kactus:
* Operating system:  Mac


## Steps to Reproduce
make a sketch folder that is broken and cannot be opened with sketch when converted to `.sketch` file.
and click `Regenerate Sketch File`.

### Expected Behavior
There should be some alert that notifies the sketch folder is broken.

### Actual Behavior
There's no error & user doesn't know there are some issues with generated folder & past commits.


## Additional Information
I think broken folder after committing & merging is also a critical issue. but I cannot reproduce it.

### Logs