Added "already exists" to YSQL known error messages

Author: qvad

src/sqlancer/yugabyte/ysql/YSQLProvider.java

@@ -259,6 +259,7 @@ private boolean isRetryableDatabaseCreationError(Exception e) {
                 || msg.contains("is being accessed by other users")
                 || msg.contains("connection has been closed")
                 || msg.contains("does not exist")
+                || msg.contains("already exists")
                 || msg.contains("Timed out waiting")
                 || msg.contains("Restarting a DDL transaction not supported");
     }