actool validate does not return context for failures

[jzelinskie]

When debugging an ACI, it would be extremely useful to see the line number or field for the error in the manifest along with the error. Currently, I'm debugging a manifest and the error I get is as follows: img.tar.gz: error validating: image manifest validation failed: ACName must contain only lower case alphanumeric characters plus "-". I now know that one of the fields which is supposed to be an ACName is not following the regex for ACName, but I do not know which field.