Assert targets at runtime

[koddsson]

We need a way to assert that the target exists in the HTML at runtime.

[Scipion]

Something like throwing a ReferenceError when findTarget returns undefiend (and findTargets returns empty Array) would do the work. I've checked that there is not throw Error in the whole project, so I assume it's not the desired solution.

I know it's an old issue, but there is any code style definition about how to implement it?