D9578: Improve error message when parttest fails due to missing pdflatex

[Henrik Fehlauer]

rkflx added a reviewer: Okular.

REPOSITORY
  R223 Okular

REVISION DETAIL
  https://phabricator.kde.org/D9578

To: rkflx, aacid, #okular
Cc: #okular, michaelweghorn, gassaf, ngraham, aacid