[SPARK-20459][SQL] JdbcUtils throws IllegalStateException: Cause already...

[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.