Fix a bug in the Google-internal rule "Fileset" (not in Bazel).

--
MOS_MIGRATED_REVID=109195426
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunction.java
index 6181a11..7684a59 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/RecursiveFilesystemTraversalFunction.java
@@ -84,7 +84,9 @@
     public final String unresolvedLink;
 
     public DanglingSymlinkException(String path, String unresolvedLink) {
-      super("Found dangling symlink: " + path + ", unresolved path: ");
+      super(
+          String.format(
+              "Found dangling symlink: %s, unresolved path: \"%s\"", path, unresolvedLink));
       Preconditions.checkArgument(path != null && !path.isEmpty());
       Preconditions.checkArgument(unresolvedLink != null && !unresolvedLink.isEmpty());
       this.path = path;