Better error message on IPC EPIPE

[ehmicky]

When sendMessage() is sent exactly:

• After the subprocess has started disconnecting
• But before it has emitted the disconnect event, set subprocess.connected false nor set subprocess.channel null

Then an EPIPE error is emitted.

Error: write EPIPE
    at target._send (node:internal/child_process:879:20)
    at target.send (node:internal/child_process:752:19)
    at node:internal/util:430:21
    at new Promise (<anonymous>)
    at bound  (node:internal/util:416:12)
    at ...
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5) {
  errno: -32,
  code: 'EPIPE',
  syscall: 'write'
}

This PR provides with a better error message instead:

Error: subprocess.sendMessage() cannot be used: the subprocess is disconnecting.
    at ...
  [cause]: Error: write EPIPE
      at target._send (node:internal/child_process:879:20)
      at target.send (node:internal/child_process:752:19)
      at node:internal/util:430:21
      at new Promise (<anonymous>)
      at bound  (node:internal/util:416:12)
      at ...
      at process.processTicksAndRejections (node:internal/process/task_queues:95:5) {
    errno: -32,
    code: 'EPIPE',
    syscall: 'write'
  }
}