Improve error message from Insert Text at Cursor brick

[grahamlangford]

User Story

As a PixieBrix mod developer, I want meaningful error messages for the Insert Text at Cursor brick in the Page Editor, so that I can debug errors myself

Motivation

See: https://pixiebrix.slack.com/archives/C0436P48QHY/p1711401954499719

[twschiller]

Potential message: "Unable to insert text at cursor. Is there an active text editor?" We should ensure we pass along the original error as the cause

[fregante]

There's no original error, this is the result of execCommand returning false. This specifically happens when there's no field focused/selected or if the field is read-only.

As seen in the last related PR, we generally don't try to get the selected/focused field before running execCommand unless necessary.

[twschiller]

There's no original error, this is the result of execCommand returning false.

The "original error" is here:

pixiebrix-extension/src/contentScript/textEditorDom.ts

Line 72 in 837ea11

throw new Error("Failed to insert text using execCommand");

. So we either need to change that error, or wrap it in InsertAtCursorEffect. I would wrap it and keep the error thrown by insertAtCursorWithCustomEditorSupport