Fix 1214: Don't crash with \relative c' \new Voice {...}

Fix 1214: Don't crash with \relative c' \new Voice {...}

Also add better warnings if the quoted music is empty
or cannot be found.

Also works if a complete voice (or a more complex structure) is
quoted, i.e. when one uses
   \addQuote "quote" \new Voice {...}
   \addQuote "quote" \relative c' \new Voice {...}

As a positive (unintended) side-effect, quoting music
with parallel sections, i.e. containing << { ...} \\ { ...} >>
now also quote those notes.