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 <ruslan.mstoi@intel.com>