& unify handlers custom errors in errors.go file

forbids use of named returned parameters

The jobs cannot be really cancelled by wpd itself. So the only way to actually
stop an ongoing render is to kill the process.
Because a single wpd instance will never run several jobs at the same time within
clad (thanks to the PooledEngine way of dispatching jobs), we can do that without
risking any later problem.
Doing that solve the problem of wpd processes taking 100% CPU for nothing.

Now the engine registers a setup and handle their own config.
The top level engine options orchestrates the engine setups.
New settings '[Engines] enabled & disabled' allow enabling or disabling given engines.