Reduce use of error-level logs

[iamed2]

If a package uses LibPQ and catches a LibPQ error, LibPQ still produces an error log. This can result in confusing logs, where caught errors show up as ERROR even though as far as the calling package is concerned, they're normal behaviour. This can't be controlled by setting the LibPQ log level, because multiple packages in an application can use LibPQ and handle different error conditions. The best solution I see for this is to just make existing calls to error just throw an exception instead.