Avoid JS error in the webui popover script

[brianhogg]

Description

Checks that LLMS.l10n.translate is defined, otherwise use a default 'Close' aria label.

Fixes #2678

How has this been tested?

Manually

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• [ ] This PR requires and contains at least one changelog file. <!-- To create a changelog yml file: npm run dev changelog add -- -i and follow the prompt. See also: https://github.com/gocodebox/lifterlms/blob/trunk/packages/dev/README.md#changelog-add -->
• [ ] My code has been tested.
• [ ] My code passes all existing automated tests.
• [ ] My code follows the LifterLMS Coding & Documentation Standards.