Makefile improvements: Assert that source message files exist before downloading from CrowdIn
Makefile improvements: Assert that source message files exist before downloading from CrowdIn

lyw07/kolibri,benjaoming/kolibri,indirectlylit/kolibri,DXCanas/kolibri,learningequality/kolibri,learningequality/kolibri,benjaoming/kolibri,indirectlylit/kolibri,DXCanas/kolibri,indirectlylit/kolibri,DXCanas/kolibri,indirectlylit/kolibri,mrpau/kolibri,lyw07/kolibri,lyw07/kolibri,benjaoming/kolibri,DXCanas/kolibri,lyw07/kolibri,mrpau/kolibri,learningequality/kolibri,mrpau/kolibri,mrpau/kolibri,learningequality/kolibri,benjaoming/kolibri