Validate "-jvm-target" value in CLI, improve message

compiler/frontend.java/src/org/jetbrains/kotlin/config/JVMConfigurationKeys.java

@@ -57,8 +57,8 @@ public class JVMConfigurationKeys {
     public static final CompilerConfigurationKey<Boolean> USE_TYPE_TABLE =
             CompilerConfigurationKey.create("use type table in serializer");
 
-    public static final CompilerConfigurationKey<Boolean> JVM_8_TARGET =
-            CompilerConfigurationKey.create("compile to jvm 8");
+    public static final CompilerConfigurationKey<JvmTarget> JVM_TARGET =
+            CompilerConfigurationKey.create("JVM bytecode target version");
 
     public static final CompilerConfigurationKey<IncrementalCompilationComponents> INCREMENTAL_COMPILATION_COMPONENTS =
             CompilerConfigurationKey.create("incremental cache provider");

compiler/frontend.java/src/org/jetbrains/kotlin/config/JvmTarget.kt

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2010-2016 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.kotlin.config
+
+enum class JvmTarget(val string: String) {
+    JVM_1_6("1.6"),
+    JVM_1_8("1.8"),
+    ;
+
+    companion object {
+        @JvmStatic
+        fun fromString(string: String) = values().find { it.string == string }
+    }
+}