Don't ignore exceptions when applying utf8mb4 conversions

It should be safe to apply them multiple times.  If an error occurs, then
there's something that is broken and needs fixing.  Don't hide the problem.

Also, don't attempt to process empty lines or comments.  Doing so could lead to
errors.  This treatment is consistent with JInstaller::parseSQLFiles().

administrator/components/com_installer/models/database.php

@@ -287,13 +287,18 @@ public function convertTablesToUtf8mb4()
 
 		foreach ($queries as $query)
 		{
-			try
-			{
-				$db->setQuery($query)->execute();
-			}
-			catch (Exception $e)
+			$query = trim($query);
+
+			if ($query != '' && $query{0} != '#')
 			{
-				// If the query fails we will go on. It probably means we've already done this conversion.
+				try
+				{
+					$db->setQuery($query)->execute();
+				}
+				catch (RuntimeException $e)
+				{
+					JFactory::getApplication()->enqueueMessage(JText::sprintf('JLIB_DATABASE_QUERY_FAILED', $e->getCode(), $e->getMessage()));
+				}
 			}
 		}
 	}