Add comments and clean up test for !1274

lib/gitlab/markdown/syntax_highlight_filter.rb

@@ -24,6 +24,8 @@ module Gitlab
         begin
           highlighted = block_code(code, language)
         rescue
+          # Gracefully handle syntax highlighter bugs/errors to ensure
+          # users can still access an issue/comment/etc.
           highlighted = "<pre>#{code}</pre>"
         end
 

spec/lib/gitlab/markdown/syntax_highlight_filter_spec.rb

@@ -4,9 +4,6 @@ module Gitlab::Markdown
   describe SyntaxHighlightFilter do
     include FilterSpecHelper
 
-    let(:project)   { create(:empty_project) }
-    let(:reference) { snippet.to_reference }
-
     it 'highlights valid code blocks' do
       result = filter('<pre><code>def fun end</code>')
       expect(result.to_html).to eq("<pre class=\"code highlight js-syntax-highlight plaintext\"><code>def fun end</code></pre>\n")