compact: Provide user with more error feedback

Provide instructions on what to do when we couldn't move the compacted
database into place.

Signed-off-by: Ben Gamari <bgamari.foss@gmail.com>
database: More error handling for compaction

We never checked to ensure that the rmtree() of the old database
succeeded.

Signed-off-by: Ben Gamari <bgamari.foss@gmail.com>