improve structure of prioritized_ output

[jacobvjk]

          Makes sense and works as expected as far as I can tell locally. My only minor concern is that rather than spitting out a warning to the console for every loanbook included in the process (which could be 1000's maybe), now it will create a new CSV in the `prioritized_loanbooks_and_diagnostics` output directory for every loanbook included in the process? Maybe in the future we can think about how to better organize the `prioritized_loanbooks_and_diagnostics` output directory.

Originally posted by @cjyetman in #240 (review)