[SPARK-20459][SQL] JdbcUtils throws IllegalStateException: Cause already initialized after getting SQLException

## What changes were proposed in this pull request?

Avoid failing to initCause on JDBC exception with cause initialized to null

## How was this patch tested?

Existing tests

Author: Sean Owen <sowen@cloudera.com>

Closes #17800 from srowen/SPARK-20459.

Author: srowen

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

@@ -652,8 +652,17 @@ object JdbcUtils extends Logging {
       case e: SQLException =>
         val cause = e.getNextException
         if (cause != null && e.getCause != cause) {
+          // If there is no cause already, set 'next exception' as cause. If cause is null,
+          // it *may* be because no cause was set yet
           if (e.getCause == null) {
-            e.initCause(cause)
+            try {
+              e.initCause(cause)
+            } catch {
+              // Or it may be null because the cause *was* explicitly initialized, to *null*,
+              // in which case this fails. There is no other way to detect it.
+              // addSuppressed in this case as well.
+              case _: IllegalStateException => e.addSuppressed(cause)
+            }
           } else {
             e.addSuppressed(cause)
           }