Improve content of the "already exists" message

[bobbingwide]

While attempting to reproduce issue #39 I experienced the following messages.

Page "My Sites" already exists. Post "Hello world!" already exists.

In the first instance the page being imported was an updated version of the "Sample Page", post ID 2. It would help if the message somehow indicated which page to delete.

In the second instance, the post ( ID 1 ) had been consigned to Trash, but the Trash had not been emptied. Again, it would help if the message indicated this.