[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.