Fixes issue #115 (logging in with invalid character results in creating a new character)

The HELLO message was replaced with two new message types, CREATE and LOGIN, which are now handled separately on the server.
Don't differentiate between invalid username or invalid password - present both cases as the same error (for security reasons).
Fix respawn button
Avoid running tryStartingGame again if we're already in the process of launching the game

This can happen if, for example, you hit enter more than once while logging in. The only visible impact seemed to be that they play button text (either "Login" or "Create") did not get restored properly (it stayed as "Loading"), but this has potential to cause other problems as well, and should be fixed.