Check whether the target document already exists prior to the user clicking "Import"

Steps to follow to reproduce:

• go to actions
• click on "Import office document"
• click "Browse" and add a doc
• in the "Target space" field add a space that already exists
• in the "Target page" field also add an existing page from that space
• click "Import"

Result: the following error will appear: "The target document already exists."

Suggestion: check whether the Target page is already a page in the wiki and if it is tell the user he needs to pick a different name for the page