### What changes were proposed in this pull request?
This PR proposes to improve the error message from Scalar iterator pandas UDF.
### Why are the changes needed?
To show the correct error messages.
### Does this PR introduce any user-facing change?
Yes, but only in unreleased branches.
```python
import pandas as pd
from pyspark.sql.functions import pandas_udf, PandasUDFType
pandas_udf('long', PandasUDFType.SCALAR_ITER)
def pandas_plus_one(iterator):
for _ in iterator:
yield pd.Series(1)
spark.range(10).repartition(1).select(pandas_plus_one("id")).show()
```
```python
import pandas as pd
from pyspark.sql.functions import pandas_udf, PandasUDFType
pandas_udf('long', PandasUDFType.SCALAR_ITER)
def pandas_plus_one(iterator):
for _ in iterator:
yield pd.Series(list(range(20)))
spark.range(10).repartition(1).select(pandas_plus_one("id")).show()
```
**Before:**
```
RuntimeError: The number of output rows of pandas iterator UDF should
be the same with input rows. The input rows number is 10 but the output
rows number is 1.
```
```
AssertionError: Pandas MAP_ITER UDF outputted more rows than input rows.
```
**After:**
```
RuntimeError: The length of output in Scalar iterator pandas UDF should be
the same with the input's; however, the length of output was 1 and the length
of input was 10.
```
```
AssertionError: Pandas SCALAR_ITER UDF outputted more rows than input rows.
```
### How was this patch tested?
Unittests were fixed accordingly.
Closes#28135 from HyukjinKwon/SPARK-26412-followup.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
4fafdcd63b