more descriptive error messages

database.d

@@ -71,6 +71,8 @@ struct Row {
 	package ResultSet resultSet;
 
 	string opIndex(size_t idx) {
+		if(idx >= row.length)
+			throw new Exception(text("index ", idx, " is out of bounds on result"));
 		return row[idx];
 	}
 

mysql.d

@@ -100,6 +100,8 @@ class MySqlResult : ResultSet {
 	private void fetchNext() {
 		assert(result);
 		auto r = mysql_fetch_row(result);
+		if(r is null)
+			throw new Exception("there is no next row");
 		uint numFields = mysql_num_fields(result);
 		uint* lengths = mysql_fetch_lengths(result);
 		string[] row;