Fix #47 by keeping checkUnit in sync with codesToStatus

Previously, checkUnit and codesToStatus could report completely different answers about whether a package built successfully or not if the failure occurs after an Installing executable X in ... message occurs. This was the case when building Agda-2.6.1.3—see #47 (closed). To avoid this problem, we now ensure that checkUnit only returns BuildSucceeded in this scenario if codesToStatus also returns PackageBuildSucceeded.