`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.