Report error when `all` does not exist in image comparison result

[twolfson]

We have a regression with image height normalization (or the using the proper paths) but it is being covered over by a silenced error. While imagemagick is outputting

 /tmp/tmp-269851mbuucw.png PNG 987x6867 987x6867+0+0 8-bit DirectClass 632KB 0.160u 0:00.160
/tmp/tmp-26985szzxaxs.png PNG 987x6720 987x6720+0+0 8-bit DirectClass 618KB 0.150u 0:00.150
compare.im6: image widths or heights differ `/tmp/tmp-269851mbuucw.png' @ error/compare.c/CompareImageCommand/962.

to stderr. We are ignoring it because we do our assertion based off of stderr.

https://github.com/uber/image-diff/blob/0.4.0/lib/image-diff.js#L52

However, we should be doing a lookup for all and throw an error containing stderr if it is not found.

[twolfson]

Another tell tale sign will be that the diffImage does not exist on disk. However, I am not sure that will always be a required parameter so we should not assert against it.

[twolfson]

Resolved in 0.5.0