Preview generation fallback (#725)

* Added preview generation fallback feature.
When a preview generation fails (often for wmv/avi files), the new code tries with less stricted (no xerror) and more time consuming options (slow+fast seek).
Fix a minor issue when stash downloads ffmpeg/ffprobe, but doesn't re-detect their paths.

pkg/manager/manager.go

@@ -145,10 +145,12 @@ The FFMPEG and FFProbe binaries should be placed in %s
 The error was: %s
 `
 			logger.Fatalf(msg, configDirectory, err)
+		} else {
+			// After download get new paths for ffmpeg and ffprobe
+			ffmpegPath, ffprobePath = ffmpeg.GetPaths(configDirectory)
 		}
 	}
 
-	// TODO: is this valid after download?
 	instance.FFMPEGPath = ffmpegPath
 	instance.FFProbePath = ffprobePath
 }