Title::newFromText will return null if the user provides an invalid title

If a user provides titles like "[[Main Page]]" or "Main Page :>>", Title::newFromText
will return null. Now it will show the same error message as if the page didn't
exist rather than causing a fatal error.

Change-Id: I6a9c7a2f6023f43cb1563b28f8e67d5262dbf61c
Merge "Title::newFromText will return null if the user provides an invalid title"