Improve error handling of mongodump when namespace doesn't exist

A customer has reported that the error received when attempting to use mongodump on an inexistent namespace was not clear. I have reproduced the scenario and even though the error is clear when the target cluster is a replica set...

2019-02-15T10:02:45.959+1100	Failed: error counting testdb.: Invalid namespace specified 'testdb.'

The error retrieved is unclear when the target is a sharded cluster:

2019-02-15T14:19:07.866+1100	Failed: error counting c5_loadtest.: '.' is an invalid character in a database name

Please improve the error handling for sharded clusters so that it explicitly inform about the invalid namespace.