Update exit code for expected early exit

[lkatalin]

Fixes #1178

The root cause of the bug was the GHA ignores an exit 0 as an early exit. This PR changes the exit code to 2 in the early exit case, and adds output that can be checked in the if-failed job to skip creating an issue if the "failure" is just an early exit.

Update: This PR splits the sync step into two:

• first the diff-main-prod step checks whether any files changed other than snapshot or timestamp, then
• the sync step executes conditionally based on whether the diff-main-prod step output an abort=true

[lkatalin]

Updates made, thanks for the suggestions. I did some testing on the syntax the step uses to succeed/fail and this seemed to work.