Guardian Souls (additional letters to hit 15)

Since it came up as possible option to me not getting my Weekly Challenge Guardian reward for Velganos, here a suggestion, that should have been in place from the start.

As long as a Soul is left uncollected, the player should not be able to leave.
It is the same QoL mechanism as for restarting, so it shouldn’t be a problem to implement.

Cheers.