Better handling of cases where man is not installed

Thanks to @floam, adapted to reduce nesting.

Works around override of command-not-found handler in previous solution
(c.f. 13e025bdb01cc4dd08463ec497a0a3495873702f).

share/functions/man.fish

@@ -1,13 +1,13 @@
+if not command -qs man
+    # see #5329 and discussion at https://github.com/fish-shell/fish-shell/commit/13e025bdb01cc4dd08463ec497a0a3495873702f
+    exit
+end
+
 function man --description "Format and display the on-line manual pages"
     # Work around the "builtin" manpage that everything symlinks to,
     # by prepending our fish datadir to man. This also ensures that man gives fish's
     # man pages priority, without having to put fish's bin directories first in $PATH
 
-    if not command -qs man
-        echo fish: (_ "Unknown command") 'man'
-        return 1
-    end
-
     set -l manpath
     if set -q MANPATH
         set manpath $MANPATH