Use Liquid::Tag#raw to clarify error message

[ashmaroli]

This is to add clarity to the error message output when the tag is used without any arguments by using Liquid's builtin Liquid::Tag#raw

Currently on master

Liquid Exception: Syntax error in tag 'gist' while parsing the following markup: Valid syntax:
{% gist user/1234567 %} {% gist user/1234567 foo.js %} {% gist 28949e1d5ee2273f9fd3 %} {% gist
28949e1d5ee2273f9fd3 best.md %} in source/_test/doc.md

With the proposed change

Liquid Exception: Syntax error in tag 'gist' while parsing the following markup: '{% gist %}'
Valid syntax: {% gist user/1234567 %} {% gist user/1234567 foo.js %} {% gist 28949e1d5ee2273f9
fd3 %} {% gist 28949e1d5ee2273f9fd3 best.md %} in source/_test/doc.md

[DirtyF]

Thanks @ashmaroli

@jekyllbot: merge +fix

[mattr-]

This PR didn't pass CI. I don't understand why it was merged.

If you're going to say, "well, it's just a lint error", then linting our codebase has no value and we should just remove linting from the CI run and stop enforcing our code style.

[DirtyF]

'Well it was just a human error".

It's fixed now: https://travis-ci.org/jekyll/jekyll-gist