ipc: report some user errors better

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.

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