install procedure: allow to check again as long as Gammu DB is missing

In DB Setup, when Gammu DB was missing, it was not possible to "check again"
it's presence after manual action from the user. The user only had the option
to continue to the next screen, which is not correct.

This adds the possibility to "check again" and forbids to continue further
until Gammu DB is installed.

application/views/main/install/database_setup.php

@@ -91,7 +91,7 @@
 </table>
 <p>&nbsp;</p>
 
-<?php if ($exception !== NULL): ?>
+<?php if ($exception !== NULL || ! $has_smsd_database): ?>
 <div align="center">
 	<?php
 	echo form_open('install/database_setup');
@@ -111,7 +111,7 @@
 	echo form_submit('submit', '‹ '.tr_raw('Previous'), 'class="button"');
 	echo form_close();
 ?>
-		<?php if ($exception === NULL): ?>
+		<?php if ($exception === NULL && $has_smsd_database): ?>
 		<?php if ($type === 'install' OR $type === 'upgrade' OR ! $this->Kalkun_model->has_table_pbk()):
 		$btn_text = 'Run Database Setup'.' ›';
 	else: