Explicitly validate that the argument to JarCreator.addDirectory exists

Follow-up to https://github.com/bazelbuild/bazel/commit/0b2352de3101e87647d083f6089246079dda0f75

PiperOrigin-RevId: 177415788
diff --git a/src/java_tools/buildjar/java/com/google/devtools/build/buildjar/jarhelper/JarCreator.java b/src/java_tools/buildjar/java/com/google/devtools/build/buildjar/jarhelper/JarCreator.java
index 097f6d7..bb8be68 100644
--- a/src/java_tools/buildjar/java/com/google/devtools/build/buildjar/jarhelper/JarCreator.java
+++ b/src/java_tools/buildjar/java/com/google/devtools/build/buildjar/jarhelper/JarCreator.java
@@ -109,6 +109,9 @@
    * @param directory the directory to add to the jar
    */
   public void addDirectory(Path directory) {
+    if (!Files.exists(directory)) {
+      throw new IllegalArgumentException("directory does not exist: " + directory);
+    }
     try {
       Files.walkFileTree(
           directory,