[core] test: Add explicit messages when a template fails its validation

[cbentejac]

Description

This PR adds explicit messages that explain why the test that checks whether a template is valid has failed. Prior to it, only compatibility issues were clearly explained.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 80.88%. Comparing base (4585522) to head (698caf3). :white_check_mark: All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2937   +/-   ##
========================================
  Coverage    80.88%   80.88%           
========================================
  Files           59       59           
  Lines         7857     7857           
========================================
  Hits          6355     6355           
  Misses        1502     1502           

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