Tuple: fix TryUnpack that could throw exceptions in some conditions, …

…instead of returning false

- fix FailLegacyTupleNotSupported to return an Exception instead of throwing

Doxense.Core/Tuples/Encoding/TupleParser.cs

@@ -1400,7 +1400,7 @@ public static T VisitNext<T>(ref TupleReader reader, Func<Slice, TupleSegmentTyp
 		[Pure, MethodImpl(MethodImplOptions.NoInlining)]
 		internal static Exception FailLegacyTupleNotSupported()
 		{
-			throw new FormatException("Old style embedded tuples (0x03) are not supported anymore.");
+			return new FormatException("Old style embedded tuples (0x03) are not supported anymore.");
 		}
 
 		#endregion