Refactor IMediaImage.Open to return error status instead of boolean.

Aaru.Images/Anex86/Read.cs

@@ -43,13 +43,13 @@ namespace Aaru.DiscImages
     public sealed partial class Anex86
     {
         /// <inheritdoc />
-        public bool Open(IFilter imageFilter)
+        public ErrorNumber Open(IFilter imageFilter)
         {
             Stream stream = imageFilter.GetDataForkStream();
             stream.Seek(0, SeekOrigin.Begin);
 
             if(stream.Length < Marshal.SizeOf<Header>())
-                return false;
+                return ErrorNumber.InvalidArgument;
 
             byte[] hdrB = new byte[Marshal.SizeOf<Header>()];
             stream.Read(hdrB, 0, hdrB.Length);
@@ -78,7 +78,7 @@ namespace Aaru.DiscImages
 
             _anexImageFilter = imageFilter;
 
-            return true;
+            return ErrorNumber.NoError;
         }
 
         /// <inheritdoc />