UX issue: launching partially-updated games

[gajop]

When the rapid registry gets updated, but game isn't fully downloaded, trying to launch the game without finishing the update will fail as the tagged version won't exist.

Launcher should either be prevented to start or it should start using the previous rapid version (which would require us to a keep a history of rapid tags, e.g. history of byar:test, chobby:test, etc.)

(moved from spring/pr-downloader#129, zzz)

[gajop]

trying to start without downloading:
http://logs.springrts.com/logfiles/545/
http://logs.springrts.com/logfiles/565/
http://logs.springrts.com/logfiles/563/
http://logs.springrts.com/logfiles/590/
http://logs.springrts.com/logfiles/583/
http://logs.springrts.com/logfiles/590/
http://logs.springrts.com/logfiles/591/
http://logs.springrts.com/logfiles/597/
http://logs.springrts.com/logfiles/598/
http://logs.springrts.com/logfiles/532/
http://logs.springrts.com/logfiles/531/
http://logs.springrts.com/logfiles/509/