Show line number when validation fails

[ben-edna]

When a manifest is validated, it only shows the path to the element. Not the actual line number.

[ben-edna]

Not directly supported by pykwalify: Grokzen/pykwalify#33
But maybe through https://stackoverflow.com/a/53647080/1149326 something should be possible.

[spoorcc]

If we go for yamale then we should look at 23andMe/Yamale#117