for example if the db path doesn't exist, the forked process stops almost immediately, but there is no indication outside the log file specified that this happened. it would be a nice-to-have to get that.

it might be a bit tricky to do this with a universality to it. three approaches? :
1) fork later. then more has happened that could have gone wrong.
2) after forking, have the forking parent wait for a sentinel state from the child either that it is ok or not ok or still in initialization. once out of initialization the parent stops regardless, but perhaps with something to report.
3) just pre-check some very common errors like no data directory – partial solution but trivial to do

this brings up an interesting question of do we want an invocation with --fork to not return until init is completed anyway? i could see some advantages to that. i.e. i might have a script that after starting a mongod then wants to initiate the replica set; but maybe the mongod hasn't even started yet so i have to wait somehow.