#3809 font problem with SVG backend

[Anonymous]

Originally created by: *anonymous

Originally created by: lemzw...@googlemail.com

This snippet foo.ly

  \version "2.19.0"

  {
    \override Staff.TimeSignature.font-name = "New Century Schoolbook"
    \time 2/4
    a2
  }

processed with

  lilypond -dbackend=svg foo.ly

aborts with

  warning: cannot find SVG font #f
  /usr/local/share/lilypond/2.19.0/scm/output-svg.scm:558:7:
    In procedure string-null?
    in expression (string-null? expr):
  /usr/local/share/lilypond/2.19.0/scm/output-svg.scm:558:7:
    Wrong type argument in position 1 (expecting string):
    #<unspecified>

PDF output is just fine.

[Federico Bruni]

https://lists.gnu.org/archive/html/lilypond-user/2015-11/msg00575.html

Andrew asks if the warning message could be improved so you can see immediately which particular override (in a big and complex score) is causing the error.
The error message should point to the specific line in the input and/or print the font name which cannot be found.