Assert `compare's stderr` contains 'all'

As reported in #8, when `image-diff` has an internal error inside of a `compare` call, it lies and says the images are not equal. To repair this behavior, we are searching for `all` inside of `stderr` (where `compare` writes to).

In this PR:

- Add search for `all` inside of `stderr`
    - If `all` is not found, we assume it is an internal error and callback with `stderr` output
- Add helpful comments