add to docs (#8212)

Author: cycp

developers/topics/opcodes-and-status-codes.mdx

@@ -323,6 +323,7 @@ Along with the HTTP error code, our API can also return more detailed error code
 | 50173  | You cannot send voice messages in this channel.                                                                                                                                                                       |
 | 50178  | The user account must first be verified                                                                                                                                                                               |
 | 50192  | The provided file does not have a valid duration.                                                                                                                                                                     |
+| 50278  | Cannot send messages to this user                                                                                                                                                                                     |
 | 50600  | You do not have permission to send this sticker.                                                                                                                                                                      |
 | 60003  | Two factor is required for this operation                                                                                                                                                                             |
 | 80004  | No users with DiscordTag exist                                                                                                                                                                                        |