[Reland][Extensions] Clarify error for attempts to access a discarded page.

This is a reland of https://crrev.com/c/6918371. That CL was reverted
due to the test being flaky due to a race condition where discarding a
tab would be fully removed before the `chrome.scripting.executeScript()`
call is processed. This would create a condition where the tab ID is no
longer valid, leading to the "No tab with id" error. Some platforms can
be more aggressive in reclaiming resources for discarded tabs, so we
should make the test more robust to accept either error.

Original CL Description:

Previously, trying to inject a script into a discarded tab would result
in a "Cannot access contents of the page. Extension manifest must
request permission to access the respective host." error, but this is
untrue for accessing discarded pages (page cannot be accessed due to the
page being discarded, not because the extension does not have necessary
host permissions).

Bug: 40875684

Change-Id: Ie3ae73495d3129c8c37becf645a77bab06d2c6ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6937409
Reviewed-by: Tim <tjudkins@chromium.org>
Reviewed-by: Emilia Paz <emiliapaz@chromium.org>
Commit-Queue: Eva Su <evasu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1516374}