Select template: inform user of his mistake in a more friendly way

When user selects a json file with wrong resource instead of template, he should be informed of his mistake in a more friendly way.
It is a ClassCastException now that user sees in an Error dialog, and he may be confused if it is his mistake or an application bug. Let's not log, nor show exception in this case, but just display what kind of resource user selected and what resource was expected.