Use Liquid::Tag#raw to clarify error message (#73)

Merge pull request 73

Author: ashmaroli

lib/jekyll-gist/gist_tag.rb

@@ -25,7 +25,7 @@ def render(context)
           raise ArgumentError, <<~ERROR
             Syntax error in tag 'gist' while parsing the following markup:
 
-              #{@markup}
+              '{% #{raw.strip} %}'
 
             Valid syntax:
               {% gist user/1234567 %}

spec/gist_tag_spec.rb

@@ -168,7 +168,7 @@
       let(:gist) { "" }
 
       it "raises an error" do
-        expect(-> { output }).to raise_error
+        expect(-> { output }).to raise_error(ArgumentError)
       end
     end
   end