new_variant: improve error handling in private shell scripts

Update test instructions for the shell scripts in private overlays.

BUG=b:149701259
TEST=See detailed test steps in testdata/error_recovery_test.md

Change-Id: I46325948ff939611bc400f6d533b809912d9aaa6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2422641
Tested-by: Paul Fagerburg <pfagerburg@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Auto-Submit: Paul Fagerburg <pfagerburg@chromium.org>
1 file changed