init.cpp: log fee estimates filename on error

- small changes to Shutdown(), buffer __func__, which is now used in
  all LogPrintf() calls and format for better readability
- order using namespace alpabetically
NAK on this change.

The file will not exist the first time you run with the fee estimation code, and that is perfectly OK and expected.

Things that are Perfectly OK and expected should not be logged; that will just make people ask us why they are getting a "failed" in their debug.log.