Escape error output from Prettier

[mattpass]

Right now if Prettier finds an error when formatting we output the error line and 2 lines above and below. If that contains HTML, JS etc that is what's output in the output pane.

Simply escape the output in HTML context.

[mattpass]

The HTML is now escaped, plus outputMsg function now outputs newest item at top and placeholder text is cleared on first output: 134f3de