Add more informative error when a layer fails specification;  Add error message when trying to unzip a file that does not exist

Feature/sgct json

* Add some missing renderables
* Tiny refactor of documentation identifiers and log category keys

For consistency. This has bothered me for quite a while

Don't think it should be a breaking change, as the keys in any potential assets are obtained from the codegen/documentation parameters

And make property title formatting more consistent

closes #1822 

Feature/tileproviders

* "newPosition" is now considered to be the item's new position (index) in the list
* also update the documentation a little