Speaker Test: Show playback errors in the UI

Errors produced when trying to play a sound were being ignored. Show them in the spaker test UI, so it can help users to find their cause.

The error strings are provided directly by libcanberra (ca_strerror), so i18n will be probably missing for them. Also, I couldn't find the full list of error strings to check their usefulness.

CCBUG: 451995

I know, "Error: Success" 😄 I had to cheat a bit to test the changes. Some test on actual errors, if you know how to provoke them, would be also cool.