Fix Wiki migration task and add more test coverage.

There was an error in the #extract_attributes_from_page method that
caused an exception when checking to see if a page named "Home"
already exists.

The check occurs to handle the renaming of the main index page to
"Home" to match the Gollum standard. If there is already a page
called "Home" then the migrator will leave that page and create
the Index page as usual. Users will need to manually rename their
old "Home" page to something else then rename their "Index" page
to "Home". Fortunately, I would be highly surprised if this case
ever comes up.

I also added more verbosity to the logging so if an error does
occur, it will be easier to track down which Wiki page is causing
the problem.
Merge pull request #3240 from DanKnox/fix_wiki_migrator

Fix Wiki migration task and add more test coverage.