packaging: print more specific error messages when builddeb fails

contrib/packaging/builddeb

@@ -49,7 +49,7 b' trap "if [ \'$CLEANUP\' ] ; then rm -r \'$P'
 set -u
 
 if [ ! -d .hg ]; then
-    echo 'You are not inside a Mercurial repository!' 1>&2
+    printf "You are inside %s, which is not the root of a Mercurial repository\n" $(pwd) 1>&2
     exit 1
 fi
 
@@ -71,7 +71,7 b' changelog=debian/changelog'
 
 if [ "$BUILD" ]; then
     if [ -d debian ] ; then
-        echo "Error! debian control directory already exists!"
+        printf "Error! debian control directory already exists at %s/debian\n" $(pwd)
         exit 1
     fi