Previously, `aio/aio-builds-setup/scripts/travis-preverify-pr.sh` was supposed
to exit with 1 if a PR did not meet the preconditions and 2 if an error occurred
during pre-verification.
It relied on the exit codes of the node script that did the actual work, but
didn't account for errors that would be thrown in the `sh` script itself (e.g.
if the node script was not available). This caused such errors to appear as
non-verified PRs, instead of real errors that should fail the build.
This commit swaps the exit codes, so that now a 2 means non-verified PR and 1
designates an error.
Georgios Kalpakas