message: Minor improvement, saying why the object is invalid.
diff: @@ -1004,7 +1004,7 @@ PjRtBuffer::GetBufferForHoldLocked(ScopedHold::Type type) {
  // acquiring any other kind of hold.
  WaitForOutstandingDonationHold();
  if (device_buffer_ == nullptr) {
  - return InvalidArgument("Hold requested on invalid buffer");
  + return InvalidArgument("Hold requested on deleted or donated buffer");
  } else {
  ++holds_[type];
  }
  @@ -1084,7 +1084,8 @@ PjRtBuffer::CopyToHostAsyncInternal(bool discard_cached_copy,
  // We can't perform any other action while a donation hold is in progress.
  WaitForOutstandingDonationHold();
  if (device_buffer_ == nullptr) {
  - return InvalidArgument("CopyToHostAsync() called on invalid buffer.");
  + return InvalidArgument(
  + "CopyToHostAsync() called on deleted or donated buffer");
  }
  if (discard_cached_copy) {
  auto it = host_values_.find(host_layout);
  @@ -1154,7 +1155,7 @@ StatusOr<std::shared_ptr<Literal>> PjRtBuffer::ToLiteral(
  TF_ASSIGN_OR_RETURN(std::shared_ptr<HostValue> host_value,
  CopyToHostAsyncInternal(discard_cached_copy, layout));
  if (host_value == nullptr) {
  - return InvalidArgument("ToLiteral called on invalid buffer");
  + return InvalidArgument("ToLiteral called on deleted or donated buffer");
  }
  host_value->ready.WaitForNotification();
  TF_RETURN_IF_ERROR(host_value->status);
  @@ -1272,7 +1273,8 @@ StatusOr<std::unique_ptr<PjRtBuffer>> PjRtBuffer::CopyToDevice(
  // We can't perform any other action while a donation hold is in progress.
  WaitForOutstandingDonationHold();
  if (device_buffer_ == nullptr) {
  - return InvalidArgument("CopyToDevice called on invalid buffer");
  + return InvalidArgument(
  + "CopyToDevice called on deleted or donated buffer");
  }
  AcquireHoldLocked(&src_device_buffer);
  }
  @@ -1313,7 +1315,8 @@ Status PjRtBuffer::BlockHostUntilReady() {
  {
  absl::MutexLock lock(&mu_);
  if (device_buffer_ == nullptr) {
  - return InvalidArgument("BlockHostUntilReady() called on invalid buffer.");
  + return InvalidArgument(
  + "BlockHostUntilReady() called on deleted or donated buffer");
  }
  device_buffer = device_buffer_;
  }
  