Add additional help text for unsynced subs error.
Add additional help text for unsynced subs error.

https://unisubs.sifterapp.com/issue/1409/
norayr/unisubs,norayr/unisubs,ofer43211/unisubs,ReachingOut/unisubs,eloquence/unisubs,pculture/unisubs,wevoice/wesub,ReachingOut/unisubs,eloquence/unisubs,ujdhesa/unisubs,eloquence/unisubs,ofer43211/unisubs,pculture/unisubs,wevoice/wesub,ofer43211/unisubs,norayr/unisubs,wevoice/wesub,ReachingOut/unisubs,eloquence/unisubs,pculture/unisubs,wevoice/wesub,ujdhesa/unisubs,pculture/unisubs,norayr/unisubs,ujdhesa/unisubs,ujdhesa/unisubs,ReachingOut/unisubs,ofer43211/unisubs