eval: (eval false) should return an error status

This also caused (isatty < /dev/null) to return 0 since it uses eval,
and (ls | cat) to output using the classify indicator style since it
uses isatty. This is how I found the bug.

Reviewed-by: Grissiom <chaos.proton@gmail.com>

share/functions/eval.fish

@@ -21,6 +21,8 @@ function eval -S -d "Evaluate parameters as a command"
 	end
 
 	echo begin\; $argv \;end eval2_inner \<\&3 3\<\&- | . 3<&0
+	set -l res $status
 
 	status --job-control $mode
+	return $res
 end