Fixes

• Build contest tour: a way back that fails still gets its minute of tries with protection, counted from its first failure whether the player is online or not (a quit or a join no longer restarts it). After that minute each join tries once without protection and tells the player to join again if it fails (a way back refused while the server stops waits for the next join; after a restart the first join whose way back fails starts a new minute). A respawn into a world that is not loaded ends a running minute.
• A return that is done is never tried again, even when the message telling the player fails. At the end of a tour (and at a stop) a return that fails no longer keeps the others away.
• A player sent home into mid-air stays protected until they land even after a quit, a server stop or a reload; their next arrival starts the 30 seconds again. Flying players are released a few seconds after the return, and one whose flight ends in mid-air stays protected until they land.
• A challenge that hits a missing class is cancelled like any other error instead of stopping the round.
• /prost admin force tells the admin when the challenge failed to start, and a failed start no longer counts as played (the entry is still not drawn again until the pool resets).
• returns.yml gains a landing: list while someone is landing. Going back to 0.4.2 logs one "entry landing is incomplete" warning, which is harmless (the ways back still load).