Add more helpful error messages when validating compatibility date

[GregBrimble]

What this PR solves / how to test

Fixes WC-2854.

Author has addressed the following

• Tests
  • [ ] TODO (before merge)
  • [x] Tests included
  • [ ] Tests not necessary because:
• E2E Tests CI Job required? (Use "e2e" label or ask maintainer to run separately)
  • [ ] I don't know
  • [ ] Required
  • [x] Not required because: no e2e coverage
• Changeset (Changeset guidelines)
  • [ ] TODO (before merge)
  • [x] Changeset included
  • [ ] Changeset not necessary because:
• Public documentation
  • [ ] TODO (before merge)
  • [ ] Cloudflare docs PR(s):
  • [x] Documentation not necessary because: just adding better errors :)