bsd.doc.mk: Similar to r289391, make installing to a missing directory an error.

This avoids the problems fixed in r290772.

Sponsored by:	EMC / Isilon Storage Division
MFC after:	2 weeks

share/mk/bsd.doc.mk

@@ -136,11 +136,11 @@ realinstall:
 .if ${PRINTERDEVICE:Mhtml}
 	cd ${SRCDIR}; \
 	    ${INSTALL} -o ${BINOWN} -g ${BINGRP} -m ${BINMODE} \
-	    ${DOC}*.html ${DESTDIR}${BINDIR}/${VOLUME}
+	    ${DOC}*.html ${DESTDIR}${BINDIR}/${VOLUME}/
 .endif
 .for _dev in ${PRINTERDEVICE:Nhtml}
 	${INSTALL} -o ${BINOWN} -g ${BINGRP} -m ${BINMODE} \
-	    ${DFILE.${_dev}} ${DESTDIR}${BINDIR}/${VOLUME}
+	    ${DFILE.${_dev}} ${DESTDIR}${BINDIR}/${VOLUME}/
 .endfor
 
 spell: ${SRCS}