Satis removed packages that it couldn't find

Satis failed to connect to pear when it was building:

```
$ cd /srv/www/php-pkg.zedcore.com/install && export COMPOSER_HOME=/srv/www/php-pkg.zedcore.com/install && php /srv/www/php-pkg.zedcore.com/install/satis/bin/satis --no-interaction build /srv/www/php-pkg.zedcore.com/install/satis.conf /srv/www/php-pkg.zedcore.com/web
Scanning packages
Initializing PEAR repository https://pear.php.net
PEAR repository from https://pear.php.net could not be loaded. Undefined index: channel
Initializing PEAR repository https://pear.horde.org
The pear-pear.php.net/mail_mimedecode >=1.5.6 requirement did not match any package
The pear-pear.php.net/pear >=1.10.1 requirement did not match any package
Creating local downloads in '/srv/www/php-pkg.zedcore.com/web/dist'
Dumping 'phpunit/phpunit'.
Dumping 'sebastian/exporter'.
Dumping 'phpunit/phpunit'.
Dumping 'phpunit/phpunit'.
Dumping 'symfony/yaml'.
Dumping 'doctrine/instantiator'.
Dumping 'pear-pear.horde.org/horde_exception'.
Dumping 'symfony/console'.
Skipping 'phpunit/phpunit 4.7.x-dev' (is dev)
Dumping 'symfony/debug'.
```

Since it couldn't connect to PEAR, it's now removed the various pear dependencies from the static file it's generated. I think satis should be doing something different:

* If we can't connect to any one of the repositories, then something has almost certainly gone wrong and we should bail
* If we can't find one of the packages, then something has also probably gone wrong (either the developer mistyped or it was in a missing repository) so we should bail.

If this does get added, I imagine some people would want to force satis to update anyway so adding a --`force-on-failure` option is probably worth doing.