How to stop xargs on first error?

General method

xargs -n 1 sh -c '<your_command> $0 || exit 255' < input

Specific case

xargs -n 1 sh -c 'curl --silent --output /dev/null \
    --write-out "%{url_effective}: %{http_code}\n" $0 || exit 255' < pages.txt

Explanation

For every URL in pages.txt, executes sh -c 'curl ... $0 || exit 255' one by one (-n 1) forcing to exit with 255 if the command fails.

From man xargs:

If any invocation of the command exits with a status of 255, xargs will stop immediately without reading any further input. An error message is issued on stderr when this happens.