Don't push translations if json is invalid
If we have an invalid json file (say a comma leftover from a merge), json2po converts it to empty file and zanata pushes it, leading in all strings being destroyed on zanata
A security can be added by doing:
python -m json.tool < locale/en.json &&
inside the locales
section of the makefile before the json2po call.
@luc any better idea ?