[iOS] - Fix interstitial pages

[Brandon-T]

Summary

• Fix interstitial pages not showing

Resolves brave/brave-browser#36688

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[kjozwiak]

Verification PASSED on iPhone 11 running iOS 17.4 using the following build(s):

Chromium | 1.65.76 Chromium: 123.0.6312.29 (Official Build) unknown (64-bit) 
--- | ---
Revision | 9ae60e5e3a26...
OS | iOS

Using the STR/Cases outlined via brave/brave-ios#8789 (comment), reproduced the original issue using 1.61.1 (24.2.29.16) re: expired.badssl.com & non-existing websites not loading a failed interstitial page.

Using the same STR/Cases mentioned above, ensured that the failed interstitial page was being displayed as per the following:

Example	Example	Example	Example	Example

The above has also been verified via 1.62.2 as per brave/brave-ios#8789 (comment).

CCing @hffvld @Uni-verse