ci: fail the mdBook download on an HTTP error

The mdBook step kept `-sSL` while the cargo-nextest steps use `-LsSf`, so it
still had the failure mode this branch set out to remove. Without `--fail`, curl
treats an HTTP error as a successful transfer: it writes the error page to
/tmp/mdbook.tar.gz and exits 0, and `tar` then fails with "not in gzip format"
instead of reporting the 403 or 404 that actually happened. `--retry` cannot help
either, because curl never classifies the response as an error to retry.

Reproduced against a non-existent release asset: without -f, curl exits 0 having
written a 9-byte "Not Found" body and tar exits 2 with no usable message; with
-f, curl exits 22 and says "The requested URL returned error: 404". The real
asset still downloads and extracts (mdbook v0.4.40).

Reported by aws-sam-tooling-bot on #843.

Author: bnusunny

.github/workflows/docs.yaml

@@ -26,7 +26,11 @@ jobs:
       - name: Install mdBook
         run: |
           mkdir -p $HOME/bin
-          curl --retry 5 --retry-all-errors --retry-delay 5 -sSL \
+          # -f so an HTTP error is an error: without it curl writes the error page to
+          # the output file and exits 0, and tar then fails with "not in gzip format"
+          # — the same misleading failure this commit set out to remove. It is also
+          # what lets --retry see a 5xx as retryable at all.
+          curl --retry 5 --retry-all-errors --retry-delay 5 -sSLf \
             https://github.com/rust-lang/mdBook/releases/download/v0.4.40/mdbook-v0.4.40-x86_64-unknown-linux-gnu.tar.gz \
             -o /tmp/mdbook.tar.gz
           tar -xz -C $HOME/bin -f /tmp/mdbook.tar.gz