[SPARK-26673][FOLLOWUP][SQL] File Source V2: check existence of output path before delete it

## What changes were proposed in this pull request?
This is a followup PR to resolve comment: apache#23601 (review)

When Spark writes DataFrame with "overwrite" mode, it deletes the output path before actual writes. To safely handle the case that the output path doesn't exist,  it is suggested to follow the V1 code by checking the existence.

## How was this patch tested?

Apply apache#23836 and run unit tests

Closes apache#23889 from gengliangwang/checkFileBeforeOverwrite.

Authored-by: Gengliang Wang <gengliang.wang@databricks.com>
Signed-off-by: gatorsmile <gatorsmile@gmail.com>

Author: gengliangwang

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/FileWriteBuilder.scala

@@ -16,6 +16,7 @@
  */
 package org.apache.spark.sql.execution.datasources.v2
 
+import java.io.IOException
 import java.util.UUID
 
 import scala.collection.JavaConverters._
@@ -83,7 +84,9 @@ abstract class FileWriteBuilder(options: DataSourceOptions)
         null
 
       case SaveMode.Overwrite =>
-        committer.deleteWithJob(fs, path, true)
+        if (fs.exists(path) && !committer.deleteWithJob(fs, path, true)) {
+          throw new IOException(s"Unable to clear directory $path prior to writing to it")
+        }
         committer.setupJob(job)
         new FileBatchWrite(job, description, committer)