fix(css): Fix error call by adding expression

What does this MR do?

Adds an error message to @error calls. It fixes a problem when throwing an @error without an expression when using the Dart Sass interpreter.

I checked that the error generated is correctly rendered in the console:

index.js → dist/index.js...
[!] (plugin postcss) Error: xs is not a valid breakpoint for this @media query.
src/scss/gitlab_ui.scss
Error: xs is not a valid breakpoint for this @media query.
    at options.error (/Users/miguelrincon/workspace/gitlab/gitlab-ui/node_modules/node-sass/lib/index.js:291:26)

error Command failed with exit code 1.

This issue was introduced by !2414 (merged), and was discovered when upgrading design.gitlab.com at gitlab-org/gitlab-services/design.gitlab.com!2607 (merged)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

Closes #1616 (closed)