Un-localise paths in mount error messages

This is probably a little more complex than it needs to be, as we try to
handle the mounts as well (which generally don't error).

Fixes #229

src/main/java/dan200/computercraft/api/filesystem/FileOperationException.java

@@ -0,0 +1,41 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2019. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+
+package dan200.computercraft.api.filesystem;
+
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+import java.io.IOException;
+import java.util.Objects;
+
+/**
+ * An {@link IOException} which occurred on a specific file.
+ *
+ * This may be thrown from a {@link IMount} or {@link IWritableMount} to give more information about a failure.
+ */
+public class FileOperationException extends IOException
+{
+    private static final long serialVersionUID = -8809108200853029849L;
+
+    private String filename;
+
+    public FileOperationException( @Nullable String filename, @Nonnull String message )
+    {
+        super( Objects.requireNonNull( message, "message cannot be null" ) );
+        this.filename = filename;
+    }
+
+    public FileOperationException( String message )
+    {
+        super( Objects.requireNonNull( message, "message cannot be null" ) );
+    }
+
+    @Nullable
+    public String getFilename()
+    {
+        return filename;
+    }
+}