When moving dir to destination that already exists, the source is simply deleted

## Steps to reproduce:

Move a directory to a location that already exists with `clobber` set to false.

## Expected behavior

An error is passed to the callback.

## Actual behavior

The source directory is deleted and no copy is made

## Reason

`npc` does not seem to error when asked to copy a directory on top of an existing file/directory, it just fails silently.