add an "--assertExists" option to mongoexport

When using the "-c" option to mongoexport, the command will cheerily comply if exporting from a collection that does not exist (and report that 0 records were exported).

The mongodb node driver has a "strict" option that allows a user to specify that commands should fail if a collection is not yet defined. Can you add a similar option to mongoexport?

I have a use case where I'd like to be able to distinguish between a collection that does not exist, and a collection that has 0 records.