Added check for package name, so it doesn't fail so silently

Author: oc

core/src/main/java/cucumber/runtime/Runtime.java

@@ -84,7 +84,10 @@ public World newWorld(Set<String> tags) {
     // XXX: should this be ctor initialized?
     public void addStepdefScanPath(String[] packages) {
         for (String packageName : packages) {
-            backends.addAll(Resources.instantiateSubclasses(Backend.class, "cucumber.runtime", packageName));
+            if(packageName.matches("^([a-z]\\w*\\.?)+$"))
+                backends.addAll(Resources.instantiateSubclasses(Backend.class, "cucumber.runtime", packageName));
+            else
+                throw new CucumberException("Additional package isn't valid: " + packageName);
         }
     }
 }