Show error messages when open CSV/database fails

[simonw]

Currently it fails silently and logs to the console.

[simonw]

New error message display looks like this:

Using this code:

datasette-app/main.js

Lines 294 to 298 in 8963bd1

	dialog.showMessageBox({
	type: "error",
	message: "Error opening CSV file",
	detail: responseJson.error
	});