[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.

sql/hive/src/main/scala/org/apache/spark/sql/hive/client/IsolatedClientLoader.scala

@@ -220,9 +220,15 @@ private[hive] class IsolatedClientLoader(
               logDebug(s"hive class: $name - ${getResource(classToPath(name))}")
               super.loadClass(name, resolve)
             } else {
-              // For shared classes, we delegate to baseClassLoader.
+              // For shared classes, we delegate to baseClassLoader, but fall back in case the
+              // class is not found.
               logDebug(s"shared class: $name")
-              baseClassLoader.loadClass(name)
+              try {
+                baseClassLoader.loadClass(name)
+              } catch {
+                case _: ClassNotFoundException =>
+                  super.loadClass(name, resolve)
+              }
             }
           }
         }
@@ -264,7 +270,7 @@ private[hive] class IsolatedClientLoader(
           throw new ClassNotFoundException(
             s"$cnf when creating Hive client using classpath: ${execJars.mkString(", ")}\n" +
             "Please make sure that jars for your version of hive and hadoop are included in the " +
-            s"paths passed to ${HiveUtils.HIVE_METASTORE_JARS}.", e)
+            s"paths passed to ${HiveUtils.HIVE_METASTORE_JARS.key}.", e)
         } else {
           throw e
         }