manpage build: fail early when xmlto is not available

When we try to build manpages with xmlto and sed, but xmlto is
missing fail at the xmlto stage.  Otherwise, one may run `cd doc;
make' and miss the warnings like:

  xmlto: not found
  sed: hg.1: No such file or directory

and end up with empty files installed as manpages.

doc/Makefile

@@ -20,8 +20,8 b' hg.1.gendoc.txt: ../mercurial/commands.p'
 	${PYTHON} gendoc.py > $@
 
 %: %.xml
-	xmlto man $*.xml ; \
-	sed -e 's/^\.hg/\\\&.hg/' $* > $*~ ; \
+	xmlto man $*.xml && \
+	sed -e 's/^\.hg/\\\&.hg/' $* > $*~ && \
 	mv $*~ $*
 
 %.xml: %.txt