[SPARK-26412][PYTHON][FOLLOW-UP] Improve error messages in Scala iterator pandas UDF

[HyukjinKwon]

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.

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()

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.

[HyukjinKwon]

@WeichenXu123, mind taking another look when you're available?

[SparkQA]

Test build #120994 has finished for PR 28135 at commit 0c1df44.

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

[SparkQA]

Test build #120995 has finished for PR 28135 at commit 6309034.

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

[HyukjinKwon]

Merged to master and branch-3.0.