[mouse-] click on error note to run error-cell

[midichef]

This is a UI change, to let users click on the ! symbol to run error-cell. The goal is to make the error-cell command easier to discover, and easier to get to without remembering the shortcut zCtrl+E or the command name.

A drawback is that users may accidentally click on the !, thinking they're clicking on a cell. After error-cell runs they may not quite understand what has happened. To make it clearer, I've created a new kind of sheet, ErrorCellSheet, with its own guide to explain where they've come to, and how to get out. And in that sheet, the column name will be cell_error, which is more specific than the current text.

What do you think?