[chore] improve illegal declaration error message

[benmccann]

svelte-preprocess started failing in 4.9.0 when running the svelte-jester test suite. I couldn't figure out where the error message was coming from at first, but updating the error message helped me track it down. The root cause of that issue is being fixed in sveltejs/svelte-preprocess#407

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with [feat], [fix], [chore], or [docs].
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[benmccann]

Closing this in favor of svelteness/svelte-jester#76