Better logging (in particular, support for warnings and errors)

For example, during backup, we issue a warning when we attempt to export a blob that doesn't exist on disk (https://github.com/haskell/hackage-server/commit/39e050bce20bf08f7c3490b0a10ed8497be77ffc); however, these warnings can easily get lost in the rest of the noise.

We should probably be using some sort of logging library, and be able to issue warnings and errors in such a way that log monitoring tools can pick them up and notify admins.