useful debug info for DuplicatePlaceholderWarning

before: 'Duplicate placeholder found: `one`'
after:  'Duplicate {% placeholder "one" %} in template placeholder_tests/test_seven.html.'

^ i always found/felt that the scan-template-for-placeholders-thing django-cms does pretty 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.