fix unicode file-writing in svg2pdf
fix exception raising in svg2pdf
tweak inkscape configurability

now you only specify the 'inkscape' path if it's non-default,
rather than re-specifying the whole command (whole command still configurable for non-Inkscape use case).
Merge pull request #3699 from minrk/svgpdfunicode

svg2pdf unicode fix

also tweaks to exceptions and config in svg2pdf

closes #3698