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

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: I2c9e18ad1c809235172aa8ab043630e34f973af1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6918371
Reviewed-by: Tim <tjudkins@chromium.org>
Commit-Queue: Eva Su <evasu@chromium.org>
Reviewed-by: Emilia Paz <emiliapaz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1512630}