Fixed error message outputs in html page

[fp4code]

This is a reopening of pull request #40, simplified and more compliant with http://docs.django-cms.org/en/release-3.4.x/contributing/development-policies.html?highlight=rebase#squashing-commits

If a buggy snippet was inserted as a plugin, an erroneous tag <class 'django.template.base.TemplateSyntaxError'> was inserted in the page, instead of the html-escaped message.

[codecov-io]

Codecov Report

Merging #45 into master will increase coverage by 0.18%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   65.05%   65.24%   +0.18%     
==========================================
  Files          13       13              
  Lines         186      187       +1     
  Branches       16       16              
==========================================
+ Hits          121      122       +1     
  Misses         64       64              
  Partials        1        1

Impacted Files	Coverage Δ
djangocms_snippet/templatetags/snippet_tags.py	28.07% <50%> (+1.28%)	⬆️

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