Error nodes: improve text to include the reason for the failure

[adietish]

When resources cannot be loaded (bcs of ex. permissions or connection timeout), the error node only reports the failure, doesn't mention the reason. It should also mention why loading failed.

discussed in #23 (comment)

[adietish]

The exception bubbles up to TreeStructure.getChildElements(Any, ITreeStructureContribution).

    private fun getChildElements(element: Any, contribution: ITreeStructureContribution): Collection<Any> {
        return try {
            contribution.getChildElements(element)
        } catch (e: java.lang.Exception) {
            logger<TreeStructure>().warn(e)
            listOf(e)
        }
    }