[SPARK-18921][SQL] check database existence with Hive.databaseExists instead of getDatabase

## What changes were proposed in this pull request?

It's weird that we use `Hive.getDatabase` to check the existence of a database, while Hive has a `databaseExists` interface.

What's worse, `Hive.getDatabase` will produce an error message if the database doesn't exist, which is annoying when we only want to check the database existence.

This PR fixes this and use `Hive.databaseExists` to check database existence.

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #16332 from cloud-fan/minor.

(cherry picked from commit 7a75ee1c)
Signed-off-by: Yin Huai <yhuai@databricks.com>