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

[ardlema]

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

[rxin]

Jenkins, test this please.

[gatorsmile]

test this please

[SparkQA]

Test build #80266 has finished for PR 18844 at commit f8bcf47.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

Actually why do we need this? Can't you just add Error?

[gatorsmile]

Compared with the other mode names, the last one is the only one whose string name do not match the SaveMode enum names. Below is the current one.

case "overwrite" => SaveMode.Overwrite
case "append" => SaveMode.Append
case "ignore" => SaveMode.Ignore
case "error" | "default" => SaveMode.ErrorIfExists

[rxin]

Ok makes sense. LGTM.

[gatorsmile]

Thanks! Merging to master