Backlight OCR: Informs the user if ScreenAI init failed or no OCR results

This patch produces appropriate messaging to
inform the user when:
1. The Screen AI library providing the oCR Service
has failed to download or initialize.
2. A page has returned no OCR results.

AX-Relnotes: Users will be informed if ScreenAI fails or no OCR results.
Bug: 342563905
Change-Id: Icdfe9b2cdcfac44b2551f19a774257db5f6886c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5671001
Reviewed-by: Kyungjun Lee <kyungjunlee@google.com>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Auto-Submit: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1322841}