Add FileExistsException

StabilityMatrix.Core/Exceptions/FileExistsException.cs

@@ -0,0 +1,13 @@
+namespace StabilityMatrix.Core.Exceptions;
+
+public class FileTransferExistsException : IOException
+{
+    public string SourceFile { get; }
+    public string DestinationFile { get; }
+
+    public FileTransferExistsException(string source, string destination)
+    {
+        SourceFile = source;
+        DestinationFile = destination;
+    }
+}