Doc exception handling fun

src/crypto/store/indexeddb-crypto-store-backend.js

@@ -359,7 +359,14 @@ function createDatabase(db) {
     outgoingRoomKeyRequestsStore.createIndex("state", "state");
 }
 
+/*
+ * Aborts a transaction with a given exception
+ * The transaction promise will be rejected with this exception.
+ */
 function abortWithException(txn, e) {
+    // We cheekily stick our exception onto the transaction object here
+    // We could alternatively make the thing we pass back to the app
+    // an object containing the transaction and exception.
     txn._mx_abortexception = e;
     txn.abort();
 }

src/crypto/store/indexeddb-crypto-store.js

@@ -302,7 +302,11 @@ export default class IndexedDBCryptoStore {
      *     transaction object: an opaque object that should be passed
      *     to store functions.
      * @return {Promise} Promise that resolves with the result of the `func`
-     *     when the transaction is complete
+     *     when the transaction is complete. If the backend is
+     *     async (ie. the indexeddb backend) any of the callback
+     *     functions throwing an exception will cause this promise to
+     *     reject with that exception. On synchronous backends, the
+     *     exception will propagate to the caller of the getFoo method.
      */
     doTxn(mode, stores, func) {
         return this._connect().then((backend) => {