clear sync data on toggling LL,also throw spec. error and delegate clear

the sync data needs to be cleared toggling in both directions:
 not LL -> LL: you want to get rid of all the excess state in the
               sync data to get the RAM benefits
 LL -> not LL: you want to fill the sync data state again
               because getOutOfBandMembers won't be called

src/client.js

@@ -44,6 +44,7 @@ const ContentHelpers = require("./content-helpers");
 
 import ReEmitter from './ReEmitter';
 import RoomList from './crypto/RoomList';
+import {InvalidStoreError} from './errors';
 
 
 const LAZY_LOADING_MESSAGES_FILTER = {
@@ -3116,12 +3117,10 @@ MatrixClient.prototype.startClient = async function(opts) {
         }
     }
     // need to vape the store when enabling LL and wasn't enabled before
-    if (opts.lazyLoadMembers) {
-        const shouldClear = await this._shouldClearSyncDataIfLL();
-        if (shouldClear) {
-            await this.store.deleteAllData();
-            throw new Error("vaped the store, you need to resync");
-        }
+    const shouldClear = await this._shouldClearSyncDataIfLLToggled(opts.lazyLoadMembers);
+    if (shouldClear) {
+        const reason = InvalidStoreError.TOGGLED_LAZY_LOADING;
+        throw new InvalidStoreError(reason);
     }
     if (opts.lazyLoadMembers && this._crypto) {
         this._crypto.enableLazyLoading();
@@ -3140,18 +3139,19 @@ MatrixClient.prototype.startClient = async function(opts) {
     this._syncApi.sync();
 };
 
-/** @return {bool} need to clear the store when enabling LL and wasn't enabled before? */
-MatrixClient.prototype._shouldClearSyncDataIfLL = async function() {
-    let hadLLEnabledBefore = false;
+/** @return {bool} need to clear the store when toggling LL compared to previous session? */
+MatrixClient.prototype._shouldClearSyncDataIfLLToggled = async function(lazyLoadMembers) {
+    lazyLoadMembers = !!lazyLoadMembers;
+    // assume it was turned off before
+    // if we don't know any better
+    let lazyLoadMembersBefore = false;
     const isStoreNewlyCreated = await this.store.isNewlyCreated();
     if (!isStoreNewlyCreated) {
         const prevClientOptions = await this.store.getClientOptions();
         if (prevClientOptions) {
-            hadLLEnabledBefore = !!prevClientOptions.lazyLoadMembers;
-        }
-        if (!hadLLEnabledBefore) {
-            return true;
+            lazyLoadMembersBefore = !!prevClientOptions.lazyLoadMembers;
         }
+        return lazyLoadMembersBefore !== lazyLoadMembers;
     }
     return false;
 };

src/errors.js

@@ -0,0 +1,23 @@
+// can't just do InvalidStoreError extends Error
+// because of http://babeljs.io/docs/usage/caveats/#classes
+function InvalidStoreError(reason) {
+    const message = `Store is invalid because ${reason}, please delete all data and retry`;
+    const instance = Reflect.construct(Error, [message]);
+    Reflect.setPrototypeOf(instance, Reflect.getPrototypeOf(this));
+    instance.reason = reason;
+    return instance;
+}
+
+InvalidStoreError.TOGGLED_LAZY_LOADING = "TOGGLED_LAZY_LOADING";
+
+InvalidStoreError.prototype = Object.create(Error.prototype, {
+  constructor: {
+    value: Error,
+    enumerable: false,
+    writable: true,
+    configurable: true
+  }
+});
+Reflect.setPrototypeOf(InvalidStoreError, Error);
+
+module.exports.InvalidStoreError = InvalidStoreError;

src/matrix.js

@@ -34,6 +34,8 @@ module.exports.SyncAccumulator = require("./sync-accumulator");
 module.exports.MatrixHttpApi = require("./http-api").MatrixHttpApi;
 /** The {@link module:http-api.MatrixError|MatrixError} class. */
 module.exports.MatrixError = require("./http-api").MatrixError;
+/** The {@link module:errors.InvalidStoreError|InvalidStoreError} class. */
+module.exports.InvalidStoreError = require("./errors").InvalidStoreError;
 /** The {@link module:client.MatrixClient|MatrixClient} class. */
 module.exports.MatrixClient = require("./client").MatrixClient;
 /** The {@link module:models/room|Room} class. */