Revert the change from impossible() to panic() that I made recently
and add code to allow continuing to succeed. The raw_print issued
there doesn't seem to go anywhere, but the game switches from tiles
map to text map and runs sanely.
Also, there was a second place that initialized the text map to all
spaces rather than S_unexplored. Change it to call the routine that
was already fixed for this.
635d98d09c