`hls-notes-plugin` crashes instead of reporting no locations

[fendor]

The hls-notes-plugin reports an error instead of no locations when it can't find the note in question.

Rather, we should just show the same thing like when "Goto definition" fails.

~~Slightly unrelated, I don't know why it fails to find the note, as it does exist, but that's another question.~~ The note Note [Shutdown] is ill-formatted.