The GRR server's handling of misnamed config options needs to be more user friendly

Currently, if an unrecognized config option is provided in the writeback file, restarting the GRR server components causes the writeback to be renamed by attaching a '.bak' suffix. After this happens though, the server components cannot start, because the writeback is not initialized:

```text
Traceback (most recent call last):
File "/usr/share/grr-server/bin/grr_console", line 11, in
sys.exit(Console())
File "/usr/share/grr-server/local/lib/python2.7/site-packages/grr_response_server/distro_entry.py", line 14, in Console
flags.StartMain(console.main)
File "/usr/share/grr-server/local/lib/python2.7/site-packages/grr_response_core/lib/flags.py", line 147, in StartMain
main(sys.argv)
File "/usr/share/grr-server/local/lib/python2.7/site-packages/grr_response_server/bin/console.py", line 107, in main
server_startup.Init()
File "/usr/share/grr-server/local/lib/python2.7/site-packages/grr_response_server/server_startup.py", line 88, in Init
raise RuntimeError("Config not initialized, run \"grr_config_updater"
RuntimeError: Config not initialized, run "grr_config_updater initialize". If the server is already configured, add "Server.initialized: True" to your config.
```