Use correct ParseError

[Caedan]

Description

Issue: #5275

Currently, the user receives a confusing error messages reading unexpected expected def or export keyword, when it should just read expected def or export keyword. This can be fixed by using ParseError::ExpectedKeyword and removing the word "expected" in the error message.

Tests

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --all --all-features -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo build; cargo test --all --all-features to check that all the tests pass

[sophiajt]

thanks!