useful debug info for DuplicatePlaceholderWarning

before: 'Duplicate placeholder found: `one`'
after:  'Duplicate {% placeholder "one" %} in template placeholder_tests/test_seven.html.'

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^ i always found/felt that the scan-template-for-placeholders mechanism django-cms does to be preeetty magic.  One example being, the admin UI-- things won't pop up there if you forget to put a {% placeholder ... %} in the template etc.

Soooo this patch just gives a developer an immediate place to look at to find a fix.  It's probably more of a problem on large projects where there are many many templates etc.