[SPARK-18517][SQL] DROP TABLE IF EXISTS should not warn for non-existing tables

## What changes were proposed in this pull request?

Currently, `DROP TABLE IF EXISTS` shows warning for non-existing tables. However, it had better be quiet for this case by definition of the command.

**BEFORE**
```scala
scala> sql("DROP TABLE IF EXISTS nonexist")
16/11/20 20:48:26 WARN DropTableCommand: org.apache.spark.sql.catalyst.analysis.NoSuchTableException: Table or view 'nonexist' not found in database 'default';
```

**AFTER**
```scala
scala> sql("DROP TABLE IF EXISTS nonexist")
res0: org.apache.spark.sql.DataFrame = []
```

## How was this patch tested?

Manual because this is related to the warning messages instead of exceptions.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #15953 from dongjoon-hyun/SPARK-18517.

(cherry picked from commit ddd02f50)
Signed-off-by: Andrew Or <andrewor14@gmail.com>

sql/core/src/main/scala/org/apache/spark/sql/execution/command/ddl.scala

@@ -28,7 +28,7 @@ import org.apache.hadoop.mapred.{FileInputFormat, JobConf}
 
 import org.apache.spark.sql.{AnalysisException, Row, SparkSession}
 import org.apache.spark.sql.catalyst.TableIdentifier
-import org.apache.spark.sql.catalyst.analysis.Resolver
+import org.apache.spark.sql.catalyst.analysis.{NoSuchTableException, Resolver}
 import org.apache.spark.sql.catalyst.catalog._
 import org.apache.spark.sql.catalyst.catalog.CatalogTypes.TablePartitionSpec
 import org.apache.spark.sql.catalyst.expressions.{Attribute, AttributeReference}
@@ -202,6 +202,7 @@ case class DropTableCommand(
       sparkSession.sharedState.cacheManager.uncacheQuery(
         sparkSession.table(tableName.quotedString))
     } catch {
+      case _: NoSuchTableException if ifExists =>
       case NonFatal(e) => log.warn(e.toString, e)
     }
     catalog.refreshTable(tableName)