Fix null deref in endpoint fetcher in error cases

This patch simply ensures the unique_ptr with the response string in
the endpoint fetcher callback is actually set before trying to
dereference it. The message that is sent in this case matches the
pattern in the rest of the callback where we instead send "There was a
response error".

Bug: 1408080
Change-Id: I378c5a8ce3699a4aad908fab18b39cf147cef970
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4326054
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Ayman Almadhoun <ayman@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1115684}