DevKit: fix misleading error message

plugins/devkit/resources/org/jetbrains/idea/devkit/DevKitBundle.properties

@@ -38,6 +38,7 @@ success.deployment.message.all=All Plugins Successfully Prepared For Deployment
 
 ##Idea Sdk
 sdk.title=IntelliJ Platform Plugin SDK
+jdk.specification=Please configure JDK
 sandbox.specification=Please configure the sandbox
 sandbox.home=Sandbox Home
 sandbox.home.label=Sandbox Home:

plugins/devkit/src/projectRoots/Sandbox.java

@@ -70,9 +70,12 @@ public class Sandbox implements ValidatableSdkAdditionalData {
 
   @Override
   public void checkValid(SdkModel sdkModel) throws ConfigurationException {
-    if (StringUtil.isEmpty(mySandboxHome) || getJavaSdk() == null) {
+    if (StringUtil.isEmpty(mySandboxHome)) {
       throw new ConfigurationException(DevKitBundle.message("sandbox.specification"));
     }
+    if (getJavaSdk() == null) {
+      throw new ConfigurationException(DevKitBundle.message("jdk.specification"));
+    }
   }
 
   void readExternal(Element element) throws InvalidDataException {