Assert `compare's stderr` contains 'all'

[twolfson]

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

[twolfson]

/cc @mlmorg @Raynos

[mlmorg]

:+1:

[Raynos]

looks good