api/msgpack-rpc: Improve error infrastructure

- Add error type information to `Error`
- Rename `set_api_error` to `api_set_error` for consistency with other api_*
  functions/macros.
- Refactor the api_set_error macro to accept formatted strings and error types
- Improve error messages
- Wrap error messages with gettext macro
- Refactor msgpack-rpc serialization to transform Error instances into [type,
  message] arrays
- Add error type information to API metadata
- Normalize nvim->client and client->nvim error handling(change
  channel_send_call to accept an Error pointer instead of the `errored` boolean
  pointer)
- Use macro to initialize Error structures

src/nvim/api/tabpage.c

@@ -116,7 +116,7 @@ Window tabpage_get_window(Tabpage tabpage, Error *err)
 /// @return true if the tab page is valid, false otherwise
 Boolean tabpage_is_valid(Tabpage tabpage)
 {
-  Error stub = {.set = false};
+  Error stub = ERROR_INIT;
   return find_tab_by_handle(tabpage, &stub) != NULL;
 }