Improve consistency of linter rules messages

[felipecrs]

I spend some time reviewing the messages of these rules, and I found some little inconsistencies in the style. In my opinion this PR improves them.

[felipecrs]

Some patterns I came up with:

1. Always refer to keywords using uppercase (e.g. FROM instead of from).
2. Only quote strings if their content is unknown (i.e. entered by the user) (e.g. a stage name '%s'), or when otherwise the sentence would be confusing ('as' keyword instead of as keyword).
3. Always use backticks ` to refer to snippets

[thompson-shaun]

@felipecrs I think the test cases will also need to be updated. 🙏

[felipecrs]

For sure @thompson-shaun, I will get around to doing it soon. Please leave this on standby until I update it. Sorry.

[thompson-shaun]

No worries! Just wanted to call out in case it got lost in the pile. Thank you for the contribution. Reach out if you need assistance.