FFmpeg: Create, use av_err2string() (#689)

- Previously 'av_make_error_string' was defined in FFmpegUtilities.h
  for the sole purpose of redefining `av_err2str()` as a call to that
  function. `av_err2str()` was then used in our code, often in string
  contexts where its output was cast to `std::string`.
- Since that was excessively circular, instead the function is named
  `av_err2string()`, and it's used directly in contexts where a
  std::string is expected.
- `av_err2str()` is still #defined as `av_err2string(...).c_str()`

src/FrameMapper.cpp

@@ -828,7 +828,9 @@ void FrameMapper::ResampleMappedAudio(std::shared_ptr<Frame> frame, int64_t orig
 
 	if (error_code < 0)
 	{
-		ZmqLogger::Instance()->AppendDebugMethod("FrameMapper::ResampleMappedAudio ERROR [" + (std::string)av_err2str(error_code) + "]", "error_code", error_code);
+		ZmqLogger::Instance()->AppendDebugMethod(
+			"FrameMapper::ResampleMappedAudio ERROR [" + av_err2string(error_code) + "]",
+			"error_code", error_code);
 		throw ErrorEncodingVideo("Error while resampling audio in frame mapper", frame->number);
 	}