Show a notification when copying fails

[wheelercj]

For various reasons, Stardown is sometimes unable to read site data or write to the clipboard. A red X appears on Stardown's icon when this happens, but users may not notice this tiny error indicator. Even if they do, it's better to show a description of what went wrong. Displaying a notification solves these problems.

With this change, the readme section titled "Stardown displayed a red X and didn't do anything" can be deleted. Before deleting it, refer to it when writing the new error messages.