plugins: support failure_message in invoice and htlc_accepted hooks.

As promised in the Changelog when we converted from failcodes to messages
internally.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/plugins/fail_htlcs.py

@@ -9,9 +9,8 @@ plugin = Plugin()
 def on_htlc_accepted(onion, plugin, **kwargs):
     plugin.log("Failing htlc on purpose")
     plugin.log("onion: %r" % (onion))
-    # FIXME: Define this!
-    WIRE_TEMPORARY_NODE_FAILURE = 0x2002
-    return {"result": "fail", "failure_code": WIRE_TEMPORARY_NODE_FAILURE}
+    # WIRE_TEMPORARY_NODE_FAILURE = 0x2002
+    return {"result": "fail", "failure_message": "2002"}
 
 
 plugin.run()