pxe_utils: Increase feedback to user when fdt file is not found

extlinux.conf can set fdtdir. fdtdir look for fdt file using
information found in the enviroment variable. The function does
not report any error in the case the file is not found

Scanning for bootflows in all bootdevs
Seq  Method       State   Uclass    Part  Name                      Filename
---  -----------  ------  --------  ----  ------------------------  ----------------
Scanning global bootmeth 'efi_mgr':
No EFI system partition
No EFI system partition
Failed to persist EFI variables
Scanning bootdev 'mmc@fa10000.bootdev':
  0  extlinux     ready   mmc          1  mmc@fa10000.bootdev.part_ /boot/extlinux/extlinux.conf
** Booting bootflow 'mmc@fa10000.bootdev.part_1' with extlinux
1:      am62x-sk-buildroot
Retrieving file: /boot/Image
append: console=ttyS2,115200n8 root=PARTUUID=c586a30c-0bf1-4323-aba8-779c814ee135 rw
		rootfstype=ext4 rootwait earlycon=ns16550a,mmio32,0x02800000
Retrieving file: /boot/k3-am623_ccm_m3.dtb
Skipping fdtdir /boot/ for failure retrieving dts

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Simon Glass <sjg@chromium.org>