[SPARK-16349][SQL] Fall back to isolated class loader when classes not found.

Some Hadoop classes needed by the Hive metastore client jars are not present
in Spark's packaging (for example, "org/apache/hadoop/mapred/MRVersion"). So
if the parent class loader fails to find a class, try to load it from the
isolated class loader, in case it's available there.

Tested by setting spark.sql.hive.metastore.jars to local paths with Hive/Hadoop
libraries and verifying that Spark can talk to the metastore.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #14020 from vanzin/SPARK-16349.