Review comments

Address review comments from JustAMan, Bond-009 and cvium.

MediaBrowser.MediaEncoding/Encoder/EncoderValidator.cs

@@ -48,6 +48,10 @@ namespace MediaBrowser.MediaEncoding.Encoder
 
             if (string.IsNullOrWhiteSpace(output))
             {
+                if (logOutput)
+                {
+                    _logger.LogError("FFmpeg validation: The process returned no result");
+                }
                 return false;
             }
 
@@ -55,6 +59,10 @@ namespace MediaBrowser.MediaEncoding.Encoder
 
             if (output.IndexOf("Libav developers", StringComparison.OrdinalIgnoreCase) != -1)
             {
+                if (logOutput)
+                {
+                    _logger.LogError("FFmpeg validation: avconv instead of ffmpeg is not supported");
+                }
                 return false;
             }