More fixes for addon i18n messages management.

git-svn-id: https://svn.blender.org/svnroot/bf-extensions/trunk/py/scripts/addons@4467 96b784f8-4ae3-dd11-aff7-0030488c597c
raise HeaderError if, file header is invalid.

git-svn-id: https://svn.blender.org/svnroot/bf-extensions/trunk/py/scripts/addons@4468 96b784f8-4ae3-dd11-aff7-0030488c597c