mozilla.org: don't display English for untranslated strings
Update p12n-extract and productization view to use refactored JSON
API: return 400 and better message for missing entity

Fix issue #380
Merge pull request #393 from flodolo/productization

Update p12n-extract and productization view to use refactored JSON
Merge pull request #394 from flodolo/issue380

API: return 400 and better message for missing entity
Merge pull request #390 from flodolo/untranslated_mozorg

mozilla.org: don't display English for untranslated strings
p12n_extract.py - fix behavior for broken XML
Merge pull request #396 from flodolo/p12n_broken_xml

p12n_extract.py - fix behavior for broken XML