[SPARK-4713] [SQL] SchemaRDD.unpersist() should not raise exception if it is not persisted

Unpersist a uncached RDD, will not raise exception, for example:
```
val data = Array(1, 2, 3, 4, 5)
val distData = sc.parallelize(data)
distData.unpersist(true)
```

But the `SchemaRDD` will raise exception if the `SchemaRDD` is not cached. Since `SchemaRDD` is the subclasses of the `RDD`, we should follow the same behavior.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #3572 from chenghao-intel/try_uncache and squashes the following commits:

50a7a89 [Cheng Hao] SchemaRDD.unpersist() should not raise exception if it is not persisted

sql/core/src/main/scala/org/apache/spark/sql/SchemaRDD.scala

@@ -501,7 +501,7 @@ class SchemaRDD(
   }
 
   override def unpersist(blocking: Boolean): this.type = {
-    sqlContext.uncacheQuery(this, blocking)
+    sqlContext.tryUncacheQuery(this, blocking)
     this
   }
 }

sql/core/src/test/scala/org/apache/spark/sql/CachedTableSuite.scala

@@ -49,6 +49,20 @@ class CachedTableSuite extends QueryTest {
     uncacheTable("tempTable")
   }
 
+  test("unpersist an uncached table will not raise exception") {
+    assert(None == lookupCachedData(testData))
+    testData.unpersist(true)
+    assert(None == lookupCachedData(testData))
+    testData.unpersist(false)
+    assert(None == lookupCachedData(testData))
+    testData.persist()
+    assert(None != lookupCachedData(testData))
+    testData.unpersist(true)
+    assert(None == lookupCachedData(testData))
+    testData.unpersist(false)
+    assert(None == lookupCachedData(testData))
+  }
+
   test("cache table as select") {
     sql("CACHE TABLE tempTable AS SELECT key FROM testData")
     assertCached(sql("SELECT COUNT(*) FROM tempTable"))