[SPARK-13445][SQL] Improves error message and add test coverage for Window function

## What changes were proposed in this pull request?

Add more verbose error message when order by clause is missed when using Window function.

## How was this patch tested?

Unit test.

Author: Sean Zhong <seanzhong@databricks.com>

Closes #13333 from clockfly/spark-13445.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala

@@ -1795,7 +1795,9 @@ class Analyzer(
     def apply(plan: LogicalPlan): LogicalPlan = plan transform {
       case logical: LogicalPlan => logical transformExpressions {
         case WindowExpression(wf: WindowFunction, spec) if spec.orderSpec.isEmpty =>
-          failAnalysis(s"WindowFunction $wf requires window to be ordered")
+          failAnalysis(s"Window function $wf requires window to be ordered, please add ORDER BY " +
+            s"clause. For example SELECT $wf(value_expr) OVER (PARTITION BY window_partition " +
+            s"ORDER BY window_ordering) from table")
         case WindowExpression(rank: RankLike, spec) if spec.resolved =>
           val order = spec.orderSpec.map(_.child)
           WindowExpression(rank.withOrder(order), spec)

sql/core/src/test/scala/org/apache/spark/sql/DataFrameWindowSuite.scala

@@ -110,6 +110,14 @@ class DataFrameWindowSuite extends QueryTest with SharedSQLContext {
       Row(2, 2, 1, 5.0d / 3.0d, 3, 5, 2, 3, 2, 2, 1.0d, 0.5d) :: Nil)
   }
 
+  test("window function should fail if order by clause is not specified") {
+    val df = Seq((1, "1"), (2, "2"), (1, "2"), (2, "2")).toDF("key", "value")
+    val e = intercept[AnalysisException](
+      // Here we missed .orderBy("key")!
+      df.select(row_number().over(Window.partitionBy("value"))).collect())
+    assert(e.message.contains("requires window to be ordered"))
+  }
+
   test("aggregation and rows between") {
     val df = Seq((1, "1"), (2, "1"), (2, "2"), (1, "1"), (2, "2")).toDF("key", "value")
     df.createOrReplaceTempView("window_table")