Going to try and organize the most common failure cases and make sure they make sense for each tool. Feel free to edit this description for more test cases:

bsondump
1. no bson file specified – good
2. invalid options – good
3. pointed to a non-bson file – bson files do not have a special header so there's no nicer way to determine its invalid than just going for it.

mongodump
1. unreachable db – good
2. invalid options – good
3. auth error – good
4. broken pipe – can get EOF
5. can't use repair cursor – good

mongorestore
1. unreachable db – good
2. invalid options – good
3. bad dump folder / no dump folder – could provide some kind of troubleshooting info. Overly verbose as it is.
4. restoring without drop – hammers you with errors, but I'm not sure if there is a good way around this

mongoexport
1. invalid options – good
2. broken pipe – sometimes gives "EOF"

mongoimport
1. bad headers – "2015-01-09T16:55:59.466-0500 Failed: incompatible fields found: 'a' and 'a.c'" could be more didactic
2. invalid input – good
3. no collection – prints 4 lines and assumes "" as the collection, which is a bit odd to read.

mongotop/mongostat
1. unreachable db – in mongotop, specifying a nonexistent hostname will result with: error determining if connected to mongos: error connecting to db server: no reachable servers
2. broken pipe (EOF?) – handled decently well