Add return value checks to kvstorageWrite(), necessary for recognizing storage-full...

Add return value checks to kvstorageWrite(), necessary for recognizing storage-full errors in the JS binding. speedtest1-kvvfs.html runs successfully on localStorage with --size 1 to 6 and starts failing with storage shortage at --size 7.

FossilOrigin-Name: 13839759f8f45e4eb0aa6a5052801f3964d5b2dc2e427e5e91ee6692176381eb