ffmpeg and ffprobe validation fixes (#63)

* abort building playout if any collection contains a zero-duration item

* surface errors calling ffprobe

* improve ffmpeg/ffprobe path validation

ErsatzTV/Services/SchedulerService.cs

@@ -56,7 +56,6 @@ namespace ErsatzTV.Services
             await ScanLocalMediaSources(cancellationToken);
         }
 
-
         private async Task BuildPlayouts(CancellationToken cancellationToken)
         {
             using IServiceScope scope = _serviceScopeFactory.CreateScope();