#4613 Use location in warnings for fermata on MMR

[Simon Albrecht]

:::TeX
\version "2.19.27"
{ R1\fermata } 

Parsing...
Interpreting music...
Preprocessing graphical objects...
programming error: Object is not a markup.
continuing, cross fingers
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
warning: MultiMeasureRestText has empty extent and non-empty stencil.
Layout output to `/tmp/lilypond-83gxiX'...
Converting to `document.pdf'...
Deleting `/tmp/lilypond-83gxiX'...
This object should be a markup: ()
Success: compilation successfully completed
Completed successfully in 0.8".

There is a warning and a message so a seasoned user will deduce the problem and search for the fermata on an MMR. However, the warning should be more specific: ideally it would tell the user to use \fermataMarkup and point to the input location.