No warning when creating a page that already exists

It's actually more complicated than this, and inconsistent because of that. Consider the following 2 cases:

Scenario 1:

• in the add menu (top left), choose page
• don't choose any template (if there are any template providers on that wiki) – "empty page" option that is
• fill in a space name and page name of an existing page (say Main and WebHome), submit the form
• the existing document will be opened in edit mode, with no warning nor error

Scenario 2:

• make sure that there is a template provider available on your wiki (check Administration -> Content -> Templates)
• in the add menu (top left), choose page
• choose a template from the list of templates available (NOT "empty page")
• fill in a space name and page name of an existing page (say Main and WebHome), submit the form
• a page will be displayed with an error, saying that the document already exists

Since these 2 scenarios are inconsistent, and scenario 1 is a little odd (one would expect an error or at least some sort of message when trying to create a document that already exists), I propose to implement it the following way:

If the document already exists, a message will be displayed on top of the create form and the user gets the chance to fill in the space (and page) names again.

Note that scenario 1 will happen also if one chooses Add -> Space and fills in the name of an existing space: the webhome of that space will be opened in edit mode. My fix proposal includes this case as well.