Fail if udevadm could not be run

Fixes: #668

dracut-init.sh

@@ -461,6 +461,10 @@ inst_rules_wildcard() {
 
 prepare_udev_rules() {
     [ -z "$UDEVVERSION" ] && export UDEVVERSION=$(udevadm --version)
+    if [ -z "$UDEVVERSION" ]; then
+        derror "Failed to detect udev version!"
+        return 1
+    fi
 
     for f in "$@"; do
         f="${initdir}/etc/udev/rules.d/$f"