Warning when Discourse files are missing #233
Description
So I've figured out #225 and it seems it results from the fact that the sound files for the textgrids is not being found. In that case, it was because the files ended in .WAV instead of .wav.
When importing, there probably should be a warning about which TextGrids are missing files as the import can appear to go smoothly when in fact no sound files were found at all.