[SPARK-29442][SQL] Set default mode should override the existing mode

### What changes were proposed in this pull request?

This PR aims to fix the behavior of `mode("default")` to set `SaveMode.ErrorIfExists`. Also, this PR updates the exception message by adding `default` explicitly.

### Why are the changes needed?

This is reported during `GRAPH API` PR. This builder pattern should work like the documentation.

### Does this PR introduce any user-facing change?

Yes if the app has multiple `mode()` invocation including `mode("default")` and the `mode("default")` is the last invocation. This is really a corner case.
- Previously, the last invocation was handled as `No-Op`.
- After this bug fix, it will work like the documentation.

### How was this patch tested?

Pass the Jenkins with the newly added test case.

Closes #26094 from dongjoon-hyun/SPARK-29442.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

sql/core/src/main/scala/org/apache/spark/sql/DataFrameWriter.scala

@@ -87,10 +87,9 @@ final class DataFrameWriter[T] private[sql](ds: Dataset[T]) {
       case "overwrite" => mode(SaveMode.Overwrite)
       case "append" => mode(SaveMode.Append)
       case "ignore" => mode(SaveMode.Ignore)
-      case "error" | "errorifexists" => mode(SaveMode.ErrorIfExists)
-      case "default" => this
-      case _ => throw new IllegalArgumentException(s"Unknown save mode: $saveMode. " +
-        "Accepted save modes are 'overwrite', 'append', 'ignore', 'error', 'errorifexists'.")
+      case "error" | "errorifexists" | "default" => mode(SaveMode.ErrorIfExists)
+      case _ => throw new IllegalArgumentException(s"Unknown save mode: $saveMode. Accepted " +
+        "save modes are 'overwrite', 'append', 'ignore', 'error', 'errorifexists', 'default'.")
     }
   }
 

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

@@ -2203,4 +2203,12 @@ class DataFrameSuite extends QueryTest with SharedSparkSession {
           |*(1) Range (0, 10, step=1, splits=2)""".stripMargin))
     }
   }
+
+  test("SPARK-29442 Set `default` mode should override the existing mode") {
+    val df = Seq(Tuple1(1)).toDF()
+    val writer = df.write.mode("overwrite").mode("default")
+    val modeField = classOf[DataFrameWriter[Tuple1[Int]]].getDeclaredField("mode")
+    modeField.setAccessible(true)
+    assert(SaveMode.ErrorIfExists === modeField.get(writer).asInstanceOf[SaveMode])
+  }
 }