Using the IPC with a program, it's not often obvious that a newline must be sent to terminate a command. Print a warning if the connection is closed while there is still uninterpreted data in the buffer. Print the OS reported error if reading/writing the socket fails. Print an erro if JSON parsing fails. I considered silencing write errors if the write end is closed (EPIPE), because a client might send a bunch of commands, and then close the socket without wanting to read the reply. But then, mpv disconnects without reading further commands that might still be buffered, so it's probably a good idea to always print the error.