Merge pull request #3066 from matrix-org/dbkr/olm_savesession_undecryptable_todevice_debug

Add some debugging & a debug event for decryption

spec/unit/crypto.spec.ts

@@ -254,6 +254,7 @@ describe("Crypto", function () {
                 sendToDevice: jest.fn(),
                 getKeyBackupVersion: jest.fn(),
                 isGuest: jest.fn(),
+                emit: jest.fn(),
             } as unknown as MatrixClient;
             mockRoomList = {} as unknown as RoomList;