[SPARK-21640] Add errorifexists as a valid string for ErrorIfExists save mode

## What changes were proposed in this pull request?

This PR includes the changes to make the string "errorifexists" also valid for ErrorIfExists save mode.

## How was this patch tested?

Unit tests and manual tests

Author: arodriguez <arodriguez@arodriguez.stratio>

Closes #18844 from ardlema/SPARK-21640.

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

@@ -71,7 +71,7 @@ final class DataFrameWriter[T] private[sql](ds: Dataset[T]) {
       case "overwrite" => SaveMode.Overwrite
       case "append" => SaveMode.Append
       case "ignore" => SaveMode.Ignore
-      case "error" | "default" => SaveMode.ErrorIfExists
+      case "error" | "errorifexists" | "default" => SaveMode.ErrorIfExists
       case _ => throw new IllegalArgumentException(s"Unknown save mode: $saveMode. " +
         "Accepted save modes are 'overwrite', 'append', 'ignore', 'error'.")
     }