`Push screenshots to ert-testdata` fails when no update

[eilskra]

This generates a GitHub-notification (which in turn can generate emails), could it cancel, skip or pass instead, to avoid such notification?

[berland]

This problem at least entails what is observed in:

https://github.com/equinor/ert/actions/runs/30977169704/job/92213495208?pr=14093

where there are assertion errors in the screenshot pytest code that is not related to the screenshots. These errors currently slip through since we have continue-on-error for the screenshot.

This is another type of failure compared to a failing screenshot test, and should be handled differerently.

[berland]

The screenshot workflow started to fail un-noticed in ed77de3 #13980