Added unavailable message to prevent over-checkout of Accessories

[Godmartinz]

Description

It seems it was possible to checkout if available quantity for an accessory was 0 or below. This applies a redirect and an unavailable message

This fixes Accessories, I will be checking other categories and pushing accordingly.

Fixes #12778

Type of change

Please delete options that are not relevant.

• [ x] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test A
• Test B

Test Configuration:

• PHP version:
• MySQL version
• Webserver version
• OS version

Checklist:

• I have read the Contributing documentation available here: https://snipe-it.readme.io/docs/contributing-overview
• I have formatted this PR according to the project guidelines: https://snipe-it.readme.io/docs/contributing-overview#pull-request-guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[what-the-diff]

PR Summary

• Added a new message to the language file
  A new message has been incorporated into the language file to enhance user experience.
• Checked accessory availability before checkout
  The system now verifies if an accessory is available for checkout before processing the transaction, improving overall user experience and inventory management.