publish: --error-on-failed to exit when signing/verifying fails
Since we have limited visibility into @CiaranG's signing server, it is hard to make changes to the publishing process, especially ones that might break @CiaranG's automation. So fdroid publish mostly reports success by moving an APK from unsigned/ to repo/. In some cases, we want immediate failure, like in CI. So this adds --error-on-failed for that purpose.