Export 'not found' error for .Image / .ImageIndex

[dtrudg]

When a consumer is trying to obtain an Image or ImageIndex it may be in a flow where that image is optional.

We need to export the 'not found' error so that a consumer can check whether a call has failed due to no match, or another reason.

At present we have...

errNoMatchingImage        = errors.New("no image found matching criteria")
errMultipleMatchingImages = errors.New("multiple images match criteria")

errNoMatchingIndex         = errors.New("no index found matching criteria")
errMultipleMatchingIndices = errors.New("multiple indices match criteria")

It probable makes sense to have generic no match and multiple matches errors across image and index, and export them both.