(fix): throw an error if no storage is configured

- if the localStorage default is unsupported in the current
  environment (e.g. Node), then it should throw with a specific
  error instead of continuing and unexpectedly throwing on getItem
  - will still error out, so not a "fix" in that sense, but it now will
    throw a better, more understandable error

src/index.ts

@@ -24,6 +24,12 @@ export const persist: IArgs = (name, store, options = {}) => {
   ) {
     storage = AsyncLocalStorage
   }
+  if (!storage) {
+    return Promise.reject('localStorage (the default storage engine) is not ' +
+      'supported in this environment. Please configure a different storage ' +
+      'engine via the `storage:` option.')
+  }
+
   if (!jsonify) { jsonify = true } // default to true like mobx-persist
   const whitelistDict = arrToDict(whitelist)
   const blacklistDict = arrToDict(blacklist)