Do not require hidden templates to be valid

[Blesmol]

Hidden templates were added as additional structural layer, mainly for the scenario where there the major sheet layout is changed between seasons, but where a 1:1 relation between layout and season still does not make sense. For example, PFS2 season 01 will be using layout 1, PFS2 seasons 02 and following will be using layout 2 (according to current knowledge).

But even if two seasons are using the same major layout, it may be that there are slight differences between the layouts for these seasons. Overcoming these differences is already possible by configuring the layout.yml to take the respective values from presets entries and then providing the concrete values in the seasons main yml files.

For this scenario it would not be required that the hidden layout template is valid, can be resolved or can be used to generate anything. On the contrary: If someone would then inherit such a hidden layout.yml file, she/he would be presented with error messages like Cannot find presets "downtime_number_of_lines" to provide the missing values for that concrete season.

Thus it would make sense to not perform the isValid() and resolve() actions for hidden templates and return an error message if a hidden template is used to generate output.