Improve error reporting

[ktraunmueller]

Observed

A

\usepackage{fontspec}

will cause this log output

Package: fontspec 2017/11/09 v2.6g Font selection for XeLaTeX and LuaLaTeX

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! Fatal fontspec error: "cannot-use-pdftex"
! 
! The fontspec package requires either XeTeX or LuaTeX.
! 
! You must change your typesetting engine to, e.g., "xelatex" or
! "lualatex"instead of "latex" or "pdflatex".
! 
! See the fontspec documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  
                                                  
l.44 \msg_fatal:nn {fontspec} {cannot-use-pdftex}

but Compositor does not report an error in the Messages pane.

Expected

A meaningful error message shows up the Messages pane.

[ktraunmueller]

The fontspec package uses \msg_fatal to issue this error message. No other package (in the core distro) seems to use this error reporting facility, so maybe it's not too important to handle this case (for now).