localforage: create custom KVStore instance, refactor Instance actions

app/soapbox/storage/kv_store.js

@@ -0,0 +1,24 @@
+import localforage from 'localforage';
+
+// localForage
+// https://localforage.github.io/localForage/#settings-api-config
+export const KVStore = localforage.createInstance({
+  name: 'soapbox',
+  description: 'Soapbox offline data store',
+  driver: localforage.INDEXEDDB,
+  storeName: 'keyvaluepairs',
+});
+
+// localForage returns 'null' when a key isn't found.
+// In the Redux action flow, we want it to fail harder.
+KVStore.getItemOrError = key => {
+  return KVStore.getItem(key).then(value => {
+    if (value === null) {
+      throw new Error(`KVStore: null value for key ${key}`);
+    } else {
+      return value;
+    }
+  });
+};
+
+export default KVStore;