add a template sanity_check_paths as MUST in TEMPLATE.eb

This helps in two ways:
* Less keystrokes for newcomers in easybuild, before arriving to first success AND request a quality standard
* Permits https://github.com/hpcugent/easybuild-framework/issues/602 to actually finish gracefully

Signed-off-by: Fotis Georgatos <fotis.georgatos@uni.lu>