Revert "do not throw an exception when init parse more than once, just issue a warning"

This reverts commit ddbc562.

Author: hermanliang

Parse/src/main/java/com/parse/ParsePlugins.java

@@ -26,7 +26,6 @@ class ParsePlugins {
     private static final String INSTALLATION_ID_LOCATION = "installationId";
 
     private static final Object LOCK = new Object();
-    private static final String TAG = "com.parse.ParsePlugins";
     private static ParsePlugins instance;
 
     // TODO(grantland): Move towards a Config/Builder parameter pattern to allow other configurations
@@ -38,8 +37,7 @@ static void initialize(Parse.Configuration configuration) {
     static void set(ParsePlugins plugins) {
         synchronized (LOCK) {
             if (instance != null) {
-                PLog.w(TAG, "ParsePlugins is already initialized");
-                return;
+                throw new IllegalStateException("ParsePlugins is already initialized");
             }
             instance = plugins;
         }