lavf/mux: clarify log messages when choosing oformat fails

Current log messages talk about 'suitable' output formats. This is
confusing, because it suggests that some formats are suitable, while
others are not, which is not the case.

Also, suggest possible user actions when format cannot be guessed from a
filename.

libavformat/mux.c

@@ -102,7 +102,7 @@ int avformat_alloc_output_context2(AVFormatContext **avctx, const AVOutputFormat
         if (format) {
             oformat = av_guess_format(format, NULL, NULL);
             if (!oformat) {
-                av_log(s, AV_LOG_ERROR, "Requested output format '%s' is not a suitable output format\n", format);
+                av_log(s, AV_LOG_ERROR, "Requested output format '%s' is not known.\n", format);
                 ret = AVERROR(EINVAL);
                 goto error;
             }
@@ -110,8 +110,10 @@ int avformat_alloc_output_context2(AVFormatContext **avctx, const AVOutputFormat
             oformat = av_guess_format(NULL, filename, NULL);
             if (!oformat) {
                 ret = AVERROR(EINVAL);
-                av_log(s, AV_LOG_ERROR, "Unable to find a suitable output format for '%s'\n",
-                       filename);
+                av_log(s, AV_LOG_ERROR,
+                       "Unable to choose an output format for '%s'; "
+                       "use a standard extension for the filename or specify "
+                       "the format manually.\n", filename);
                 goto error;
             }
         }