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/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;