ffmpeg: Add extra checks and error messages (#2951)

* ffmpeg: Add extra checks and error messages

This PR adds some checks and logging error messages related to the ffmpeg context creation, that will prevent users to open issues because they don't have the correct packages installed.
Close #2762

* Update FFmpegContext.cs

Ryujinx.Graphics.Nvdec.FFmpeg/FFmpegContext.cs

@@ -18,11 +18,35 @@ namespace Ryujinx.Graphics.Nvdec.FFmpeg
         public FFmpegContext(AVCodecID codecId)
         {
             _codec = ffmpeg.avcodec_find_decoder(codecId);
+            if (_codec == null)
+            {
+                Logger.Error?.PrintMsg(LogClass.FFmpeg, $"Codec wasn't found. Make sure you have the {codecId} codec present in your FFmpeg installation.");
+
+                return;
+            }
+
             _context = ffmpeg.avcodec_alloc_context3(_codec);
+            if (_context == null)
+            {
+                Logger.Error?.PrintMsg(LogClass.FFmpeg, "Codec context couldn't be allocated.");
 
-            ffmpeg.avcodec_open2(_context, _codec, null);
+                return;
+            }
+
+            if (ffmpeg.avcodec_open2(_context, _codec, null) != 0)
+            {
+                Logger.Error?.PrintMsg(LogClass.FFmpeg, "Codec couldn't be opened.");
+
+                return;
+            }
 
             _packet = ffmpeg.av_packet_alloc();
+            if (_packet == null)
+            {
+                Logger.Error?.PrintMsg(LogClass.FFmpeg, "Packet couldn't be allocated.");
+
+                return;
+            }
 
             _decodeFrame = Marshal.GetDelegateForFunctionPointer<AVCodec_decode>(_codec->decode.Pointer);
         }