update.sh generates an empty features file when processing fails

[moritz]

The script update.sh contains the following logic:

# The ~/features dir contains a checkout of
# the https://github.com/perl6/features repo
cd ~/features
git fetch -q
BEFORE=$(git rev-parse HEAD)
git checkout -q -f origin/master
if [ "$BEFORE" != "$(git rev-parse HEAD)" ]
then perl process.pl > ~/perl6.org/online/compilers/features.html
fi

When the perl process.pl fails (most recent cause: missing Perl modules), this overwrites features.html with an empty file.

It would be more appropriate to not update the page when that happens.