The Grunt build task fails because the Cordova CLI detects that the target folder of "grunt cordova:build" already exists. Somewhere in the chain the "cordova create" command gets called to create this (already existing) folder, so either it shouldn't be called if it already exists (just updated to include the proper platforms etc) or it should be removed and recreated (after which the www folder should either be linked (cordova create --link-to style) or copied)...
The Grunt build task fails because the Cordova CLI detects that the target folder of "grunt cordova:build" already exists. Somewhere in the chain the "cordova create" command gets called to create this (already existing) folder, so either it shouldn't be called if it already exists (just updated to include the proper platforms etc) or it should be removed and recreated (after which the www folder should either be linked (cordova create --link-to style) or copied)...