disc-more gracefully handle missing ffmpeg, and checkin an ffmpeg finally

BizHawk.Emulation/DiscSystem/CUE_format.cs

@@ -55,6 +55,11 @@ namespace BizHawk.DiscSystem
 						if (!File.Exists(blobPath))
 						{
 							//if it doesn't exist, then it may be encoded.
+							FFMpeg ffmpeg = new FFMpeg();
+							if (!ffmpeg.QueryServiceAvailable())
+							{
+								throw new InvalidOperationException("No decoding service was available (make sure ffmpeg.exe is available)");
+							}
 							AudioDecoder dec = new AudioDecoder();
 							byte[] buf = dec.AcquireWaveData(blobPath);
 							blob.Load(new MemoryStream(buf));

BizHawk.Emulation/DiscSystem/Decoding.cs

@@ -31,6 +31,22 @@ namespace BizHawk.DiscSystem
 			return ret;
 		}
 
+		/// <summary>
+		/// queries whether this service is available. if ffmpeg is broken or missing, then you can handle it gracefully
+		/// </summary>
+		public bool QueryServiceAvailable()
+		{
+			try
+			{
+				string stdout = Run("-version");
+				if (stdout.Contains("ffmpeg version")) return true;
+			}
+			catch
+			{
+			}
+			return false;
+		}
+
 		string Run(params string[] args)
 		{
 			args = Escape(args);