[Enhancement] Make error message more understandable if starrocks.columns contains non-existed columns

[banmoy]

What type of PR is this：

• BugFix
• Feature
• Enhancement
• Refactor
• UT
• Doc
• Tool

Which issues of this PR fixes ：

Fixes #
If starrocks.columns contains non-existed columns, the connector will throw a NPE which is not clear. We should make the error message more understandable to tell the user what's the problem.

java.lang.NullPointerException
	at com.starrocks.connector.spark.sql.schema.InferSchema.inferSchema(InferSchema.java:60)
	at com.starrocks.connector.spark.sql.StarRocksTableProvider.inferSchema(StarRocksTableProvider.java:65)
	at org.apache.spark.sql.execution.datasources.v2.DataSourceV2Utils$.getTableFromProvider(DataSourceV2Utils.scala:90)
	at org.apache.spark.sql.DataFrameWriter.getTable$1(DataFrameWriter.scala:280)
	at org.apache.spark.sql.DataFrameWriter.saveInternal(DataFrameWriter.scala:296)
	at org.apache.spark.sql.DataFrameWriter.save(DataFrameWriter.scala:247)
	at com.starrocks.connector.spark.sql.ConfigurationITTest.testDataFrameBase(ConfigurationITTest.java:178)
	at com.starrocks.connector.spark.sql.ConfigurationITTest.testWriteConfigurationBase(ConfigurationITTest.java:146)
	at com.starrocks.connector.spark.sql.ConfigurationITTest.testTransactionConfiguration(ConfigurationITTest.java:117)

Problem Summary(Required) ：

Checklist:

• I have added test cases for my bug fix or my new feature
• This pr will affect users' behaviors
• This pr needs user documentation (for new or modified features or behaviors)
• I have added documentation for my new feature or new function