feat(worker): thread sgc error alert messages

[stanleynguyen]

Problem

Campaign with a lot of errors and long error messages are hard to be compiled in a single message

Solution

Thread the error messages one to each reply

Deployment Checklist

Any tasks that need to be done on the environment before releasing of this PR (e.g. Setting environment variables, running migrations, etc). If there's no task to be done, put "N/A"

• [x] Set SGC_ALERT_API_TOKEN env var
• [x] Set SGC_ALERT_CHANNEL env var
• [x] Remove SGC_ALERT_WEBHOOK env var