rules/sphinx.mk: stop xelatex on error
rules/sphinx.mk: stop xelatex on error

This avoids the interactive prompt of xelatex on error.

snoyberg/ghc,vTurbine/ghc,sgillespie/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,vTurbine/ghc,olsner/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,snoyberg/ghc,olsner/ghc,sdiehl/ghc,vTurbine/ghc,mettekou/ghc,sgillespie/ghc,vTurbine/ghc,vTurbine/ghc,olsner/ghc,ezyang/ghc,mettekou/ghc,snoyberg/ghc,shlevy/ghc,shlevy/ghc,sgillespie/ghc,mettekou/ghc,olsner/ghc,mettekou/ghc,sdiehl/ghc,snoyberg/ghc,mettekou/ghc,sdiehl/ghc,snoyberg/ghc,vTurbine/ghc,shlevy/ghc,mettekou/ghc,ezyang/ghc,snoyberg/ghc,sgillespie/ghc,sdiehl/ghc,sgillespie/ghc,snoyberg/ghc,sgillespie/ghc,ezyang/ghc,olsner/ghc,sdiehl/ghc,sgillespie/ghc,shlevy/ghc,vTurbine/ghc,sdiehl/ghc,olsner/ghc,shlevy/ghc,mettekou/ghc