Report error if <withKotlin> is run in fork mode

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

compiler/testData/integration/ant/jvm/withKotlinFork/A.java

@@ -0,0 +1 @@
+public class A {}

compiler/testData/integration/ant/jvm/withKotlinFork/build.log.expected

@@ -0,0 +1,16 @@
+OUT:
+Buildfile: [TestData]/build.xml
+
+build:
+    [mkdir] Created dir: [Temp]/classes
+    [javac] Compiling 1 source file to [Temp]/classes
+
+ERR:
+    [javac] <withKotlin> task does not yet support the fork mode
+
+BUILD FAILED
+[TestData]/build.xml:6: Compile failed; see the compiler error output for details.
+
+Total time: [time]
+
+Return code: 1

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>