[SPARK-13286] [SQL] add the next expression of SQLException as cause

## What changes were proposed in this pull request?

Some JDBC driver (for example PostgreSQL) does not use the underlying exception as cause, but have another APIs (getNextException) to access that, so it it's included in the error logging, making us hard to find the root cause, especially in batch mode.

This PR will pull out the next exception and add it as cause (if it's different) or suppressed (if there is another different cause).

## How was this patch tested?

Can't reproduce this on the default JDBC driver, so did not add a regression test.

Author: Davies Liu <davies@databricks.com>

Closes #14722 from davies/keep_cause.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JdbcUtils.scala

@@ -17,7 +17,7 @@
 
 package org.apache.spark.sql.execution.datasources.jdbc
 
-import java.sql.{Connection, Driver, DriverManager, PreparedStatement}
+import java.sql.{Connection, Driver, DriverManager, PreparedStatement, SQLException}
 import java.util.Properties
 
 import scala.collection.JavaConverters._
@@ -289,7 +289,7 @@ object JdbcUtils extends Logging {
       }
       val stmt = insertStatement(conn, table, rddSchema, dialect)
       val setters: Array[JDBCValueSetter] = rddSchema.fields.map(_.dataType)
-          .map(makeSetter(conn, dialect, _)).toArray
+        .map(makeSetter(conn, dialect, _)).toArray
 
       try {
         var rowCount = 0
@@ -322,6 +322,17 @@ object JdbcUtils extends Logging {
         conn.commit()
       }
       committed = true
+    } catch {
+      case e: SQLException =>
+        val cause = e.getNextException
+        if (e.getCause != cause) {
+          if (e.getCause == null) {
+            e.initCause(cause)
+          } else {
+            e.addSuppressed(cause)
+          }
+        }
+        throw e
     } finally {
       if (!committed) {
         // The stage must fail.  We got here through an exception path, so