channel.c: refactor spaghetti code

channel.c: WIP remove redundant method check and added FUNC_ATTR_NONNULL_ALL macro

channel.c channel_defs.h helpers.c: added Error field to RequestEvent, added no_op handler func

channel.c: use const char* instead of string and cleanup

channel.c; channel_defs.h; helpers.c: removed error from event again; send errors directly to the channel without using handlers and events

channel.c: fixed memory leak and lint errors

api/private/dispatch.c; api/vim.c; msgpack_rpc/channel.c msgpack_rpc/helpers.c added Error* field to msgpack_get_handler_for; further refactored channel.c

channel.c:323 changed order of evaluation in if statement

channel.c: removed superflous whitespace

dispatch.c: review comment

src/nvim/api/private/dispatch.c

@@ -32,16 +32,19 @@ static void msgpack_rpc_add_method_handler(String method,
 /// @param name API method name
 /// @param name_len name size (includes terminating NUL)
 MsgpackRpcRequestHandler msgpack_rpc_get_handler_for(const char *name,
-                                                     size_t name_len)
+                                                     size_t name_len,
+                                                     Error *error)
 {
   String m = { .data = (char *)name, .size = name_len };
   MsgpackRpcRequestHandler rv =
     map_get(String, MsgpackRpcRequestHandler)(methods, m);
 
   if (!rv.fn) {
-    rv.fn = msgpack_rpc_handle_missing_method;
+    String method_name = m.size > 0 ?
+      m : cstr_as_string("<empty>");
+    api_set_error(error, kErrorTypeException, "Invalid method: %s",
+                  method_name);
   }
-
   return rv;
 }