Send proper error messages upon parser failures, use yajl for generating command replies

Fixes: #693

testcases/t/120-multiple-cmds.t

@@ -63,4 +63,12 @@ cmd 'move gibberish' for (0 .. 10);
 
 does_i3_live;
 
+################################################################################
+# regression test: an invalid command should come back with an error.
+################################################################################
+
+my $reply = cmd 'bullshit-command-which-we-never-implement meh';
+is(scalar @$reply, 1, 'got one command reply');
+ok(!$reply->[0]->{success}, 'reply has success == false');
+
 done_testing;