Mongomirror output when source and destination collections are conflicting is misleading to the user

Problem Statement/Rationale

The error message produced when running mongomirror with conflicting collections in the source and destination is highly misleading.

Steps to Reproduce

Create a source database with a single collection `foo`. Create a destination database `dev` with collections `foo` and `bar`. Setup mongomirror to mirror all collections from the source to the destination database. Results in:

15:18:38.361 Failed: could not perform initial sync because the following collections exist on both the source and destination:

15:18:38.361​ ​dev.foo 

15:18:38.361​ Please remove these collections from the destination or run mongomirror with --drop. 

Expected Results

When running --drop based on the error message I would expect only the conflicting collections to be dropped (i.e. `foo`). A more expressive/clear error message such as 

Please remove these collections from the destination or, to remove all collections from the destination, run mongomirror with --drop.

is very much needed.
h3. Actual Results

When error message 

Please remove these collections from the destination or run mongomirror with --drop.

running `--drop` then removes both foo and bar.