[SPARK-18465] Add 'IF EXISTS' clause to 'UNCACHE' to not throw exceptions when table doesn't exist

## What changes were proposed in this pull request?

While this behavior is debatable, consider the following use case:
```sql
UNCACHE TABLE foo;
CACHE TABLE foo AS
SELECT * FROM bar
```
The command above fails the first time you run it. But I want to run the command above over and over again, and I don't want to change my code just for the first run of it.
The issue is that subsequent `CACHE TABLE` commands do not overwrite the existing table.

Now we can do:
```sql
UNCACHE TABLE IF EXISTS foo;
CACHE TABLE foo AS
SELECT * FROM bar
```

## How was this patch tested?

Unit tests

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #15896 from brkyvz/uncache.

1 files changed, 4 insertions, 1 deletions

diff --git a/sql/hive/src/test/scala/org/apache/spark/sql/hive/CachedTableSuite.scala b/sql/hive/src/test/scala/org/apache/spark/sql/hive/CachedTableSuite.scala
index fc35304c80..3871b3d785 100644
--- a/sql/hive/src/test/scala/org/apache/spark/sql/hive/CachedTableSuite.scala
+++ b/sql/hive/src/test/scala/org/apache/spark/sql/hive/CachedTableSuite.scala
@@ -101,13 +101,16 @@ class CachedTableSuite extends QueryTest with SQLTestUtils with TestHiveSingleto
     sql("DROP TABLE IF EXISTS nonexistantTable")
   }
 
-  test("correct error on uncache of nonexistant tables") {
+  test("uncache of nonexistant tables") {
+    // make sure table doesn't exist
+    intercept[NoSuchTableException](spark.table("nonexistantTable"))
     intercept[NoSuchTableException] {
       spark.catalog.uncacheTable("nonexistantTable")
     }
     intercept[NoSuchTableException] {
       sql("UNCACHE TABLE nonexistantTable")
     }
+    sql("UNCACHE TABLE IF EXISTS nonexistantTable")
   }
 
   test("no error on uncache of non-cached table") {