CLI: report error if kotlinc is invoked on Java 6 or 7

#KT-17297 Fixed

compiler/preloader/src/org/jetbrains/kotlin/preloading/Preloader.java

@@ -32,6 +32,13 @@ public class Preloader {
     public static final int DEFAULT_CLASS_NUMBER_ESTIMATE = 4096;
 
     public static void main(String[] args) throws Exception {
+        String javaVersion = System.getProperty("java.specification.version");
+        if (javaVersion.equals("1.6") || javaVersion.equals("1.7")) {
+            System.err.println("error: running the Kotlin compiler under Java " + javaVersion + " is not supported. " +
+                               "Java 1.8 or later is required");
+            System.exit(1);
+        }
+
         try {
             run(args);
         }