Insufficient error message in case the selected font cannot be found by the used latex engine

when using lualatex and the file labplot_LMMono8.tex I'm getting the error message in the log file

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! fontspec error: "font-not-found"
! 
! The font "LMMono8" cannot be found.
! 
! See the fontspec documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  

which is not properly forwarded to the user in LabelWidget. The reason for this is the current implementation where we parse only the first error line starting with a '!' which fails to extract the proper message in this case.