How to catch cURL (piped through base64) download failure

I am using cURL to download files and store them in variables (via base64):

output=$(curl -L -o - $url | base64)

The problem is that I don't know how to check if the download was successful (if so, the file should be saved in a variable).

Try

if output=$(set -o pipefail; curl ... | base64); then
  # handle success
else
  # handle failure
fi