Report error if <withKotlin> is run in fork mode

Otherwise the error message is confusing, see #KT-9292

compiler/testData/integration/ant/jvm/withKotlinFork/build.xml

@@ -0,0 +1,13 @@
+<project name="Ant Task Test" default="build">
+    <taskdef resource="org/jetbrains/kotlin/ant/antlib.xml" classpath="${kotlin.lib}/kotlin-ant.jar"/>
+
+    <target name="build">
+        <mkdir dir="${temp}/classes"/>
+        <javac fork="yes" memorymaximumsize="1G" srcdir="${test.data}" destdir="${temp}/classes" includeantruntime="false">
+            <withKotlin/>
+        </javac>
+        <jar destfile="${temp}/hello.jar">
+            <fileset dir="${temp}/classes"/>
+        </jar>
+    </target>
+</project>