## What changes were proposed in this pull request?
When there are mismatched types among cases or else values in case when expression, current error message is hard to read to figure out what and where the mismatch is.
This patch simply improves the error message for mismatched types for case when.
Before:
```scala
scala> spark.range(100).select(when('id === 1, array(struct('id * 123456789 + 123456789 as "x"))).otherwise(array(struct('id * 987654321 + 987654321 as
"y"))))
org.apache.spark.sql.AnalysisException: cannot resolve 'CASE WHEN (`id` = CAST(1 AS BIGINT)) THEN array(named_struct('x', ((`id` * CAST(123456789 AS BI
GINT)) + CAST(123456789 AS BIGINT)))) ELSE array(named_struct('y', ((`id` * CAST(987654321 AS BIGINT)) + CAST(987654321 AS BIGINT)))) END' due to data
type mismatch: THEN and ELSE expressions should all be same type or coercible to a common type;;
```
After:
```scala
scala> spark.range(100).select(when('id === 1, array(struct('id * 123456789 + 123456789 as "x"))).otherwise(array(struct('id * 987654321 + 987654321 as
"y"))))
org.apache.spark.sql.AnalysisException: cannot resolve 'CASE WHEN (`id` = CAST(1 AS BIGINT)) THEN array(named_struct('x', ((`id` * CAST(123456789 AS BI
GINT)) + CAST(123456789 AS BIGINT)))) ELSE array(named_struct('y', ((`id` * CAST(987654321 AS BIGINT)) + CAST(987654321 AS BIGINT)))) END' due to data
type mismatch: THEN and ELSE expressions should all be same type or coercible to a common type, got CASE WHEN ... THEN array<struct<x:bigint>> ELSE arr
ay<struct<y:bigint>> END;;
```
## How was this patch tested?
Added unit test.
Closes#24453 from viirya/SPARK-27551.
Authored-by: Liang-Chi Hsieh <viirya@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
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