Fix Javadoc across the project

Enable failOnWarnings for Javadoc to keep them clean.

flytekit-api/src/main/java/org/flyte/api/v1/ContainerError.java

@@ -49,12 +49,18 @@ public static ContainerError create(String code, String message, Kind kind) {
 
   /**
    * Returns a simplified code for errors, so that we can provide a glossary of all possible errors.
+   *
+   * @return error code
    */
   public String getCode() {
     return code;
   }
 
-  /** Returns an abstract error kind for this error. */
+  /**
+   * Returns an abstract error kind for this error.
+   *
+   * @return error kind
+   */
   public Kind getKind() {
     return kind;
   }

jflyte-api/src/main/java/org/flyte/jflyte/api/FileSystem.java

@@ -36,7 +36,12 @@ public interface FileSystem {
   @MustBeClosed
   WritableByteChannel writer(String uri);
 
-  /** Returns Manifest for given resource, or null if it doesn't exist. */
+  /**
+   * Returns Manifest for given resource, or null if it doesn't exist.
+   *
+   * @param uri uri
+   * @return manifest
+   */
   @Nullable
   Manifest getManifest(String uri);
 }

jflyte/src/main/java/org/flyte/jflyte/ExecutionNodeCompiler.java

@@ -54,6 +54,10 @@ class ExecutionNodeCompiler {
   /**
    * Given a list of flytekit-api nodes, validates them, and determines their sequential execution
    * order.
+   *
+   * @param nodes nodes
+   * @param runnableTasks runnable tasks
+   * @return execution nodes
    */
   static List<ExecutionNode> compile(List<Node> nodes, Map<String, RunnableTask> runnableTasks) {
     List<ExecutionNode> executableNodes =
@@ -91,6 +95,9 @@ static ExecutionNode compile(Node node, Map<String, RunnableTask> runnableTasks)
    * priority. If two nodes have the same depth, relative order in input list is used. Because of
    * that, the implementation is slightly different from what you would normally see for topological
    * sorting.
+   *
+   * @param nodes nodes
+   * @return execution nodes
    */
   static List<ExecutionNode> sort(List<ExecutionNode> nodes) {
     // priority is initial order in the list, node earlier in the list