Make DBM load methods return Option<T> instead of Result<T, E>

[sr-gi]

Currently the DBM methods related to loading data return Result<T, E> where E is always dbm::Error::NotFound. It may make more sense to get rit of NotFound from the Errors enum and make the load functions return Option<T>.

[sr-gi]

@SpoonBuoy just realized this may be relevant to your current work.