scripts: net: Fix incorrect error messages
scripts: net: Fix incorrect error messages

If environment variables ZEPHYR_BASE or NET_TOOLS_BASE are unset, an
incorrect message is printed saying that "it is set, but it is not a
directory":

$ZEPHYR_BASE is unset
$ZEPHYR_BASE is set, but it is not a directory
$NET_TOOLS_BASE is unset, no net-tools found
$NET_TOOLS_BASE set, but it is not a directory

This patch fixes that issue.

Signed-off-by: Ruslan Mstoi <348dc8e8f10581f5bc17672b9c406a6d9fcc1430@intel.com>

galak/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr