Improve exceptions in Gd driver's open()

- RuntimeException more suitable (and consistent with other drivers)
- Change message as file existence is checked beforehand in checkPath() (refs #326)
Add test for opening of an invalid image file
Merge branch 'gd-open' of github.com:vlakoff/Imagine into vlakoff-gd-open
Merge branch 'vlakoff-gd-open'
Update changelog
Merge branch 'master' into develop