kcms/soundtheme: Improve error message for sound not found
This error is the most common a user can experience in the KCM,
as it will happen if a theme doesn't provide one of the preview
sounds (ex. 'freedesktop' and sound 'batery-caution')
To minimize possible confusion for users (and bug reports), offer
a more clear (and translated) string on why it's happening.
| BEFORE |  |
|---|---|
| AFTER |  |
Edited by Ismael Asensio