[SPARK-21640][SQL][PYTHON][R][FOLLOWUP] Add errorifexists in SparkR and other documentations

## What changes were proposed in this pull request?

This PR proposes to add `errorifexists` to SparkR API and fix the rest of them describing the mode, mainly, in API documentations as well.

This PR also replaces `convertToJSaveMode` to `setWriteMode` so that string as is is passed to JVM and executes:

https://github.com/apache/spark/blob/b034f2565f72aa73c9f0be1e49d148bb4cf05153/sql/core/src/main/scala/org/apache/spark/sql/DataFrameWriter.scala#L72-L82

and remove the duplication here:

https://github.com/apache/spark/blob/3f958a99921d149fb9fdf7ba7e78957afdad1405/sql/core/src/main/scala/org/apache/spark/sql/api/r/SQLUtils.scala#L187-L194

## How was this patch tested?

Manually checked the built documentation. These were mainly found by `` grep -r `error` `` and `grep -r 'error'`.

Also, unit tests added in `test_sparkSQL.R`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #19673 from HyukjinKwon/SPARK-21640-followup.