Raise SystemExit when mark migration not found

If migration is not found, "mark" was displaying an error message but
exiting normally.

The error message is still printed and dbmigrator will now exits with
code 1.

dbmigrator/commands/mark.py

@@ -24,9 +24,8 @@ def cli_command(cursor, migrations_directory='', migration_timestamp='',
                       migrations_directory, import_modules=True)}
     migration = migrations.get(migration_timestamp)
     if migration is None:
-        logger.error(
+        raise SystemExit(
             'Migration {} not found'.format(migration_timestamp))
-        return
 
     utils.mark_migration(cursor, migration_timestamp, completed)
     if not completed:

dbmigrator/tests/test_cli.py

@@ -217,18 +217,19 @@ def test_mark_as_true_already_true(self):
         self.assertIn('20160228202637   add_table         True', stdout)
         self.assertIn('20160228212456   cool_stuff        True', stdout)
 
-    @mock.patch('dbmigrator.logger.error')
-    def test_migration_not_found(self, error):
+    def test_migration_not_found(self):
         testing.install_test_packages()
         cmd = ['--db-connection-string', testing.db_connection_string]
 
         self.target(cmd + ['--context', 'package-a', 'init', '--version', '0'])
 
-        self.target(cmd + ['mark', '-t', '012345'])
-        error.assert_called_with('Migration 012345 not found')
+        with self.assertRaises(SystemExit) as cm:
+            self.target(cmd + ['mark', '-t', '012345'])
+        self.assertEqual('Migration 012345 not found', str(cm.exception))
 
-        self.target(cmd + ['mark', '-f', '012345'])
-        error.assert_called_with('Migration 012345 not found')
+        with self.assertRaises(SystemExit) as cm:
+            self.target(cmd + ['mark', '-f', '012345'])
+        self.assertEqual('Migration 012345 not found', str(cm.exception))
 
     def test_mark_as_false(self):
         testing.install_test_packages()