Print the actual error code in a couple of places in boot_get_fdt_fit().
These are FDT error codes, not errno, so printing the string is more
helpful than printing the numeric value.
The only caller of boot_get_fdt_fit() unconditionally replaces the
returned error code (fdt_noffset) with ENOENT so the actual error would
otherwise be lost.
Signed-off-by: David Lechner <dlechner@baylibre.com>