Mac OS X .pkg installer: check for target folder before installing

If the target folder exists, let the user know and give up. (This
seems safer than trying to delete the folder, since there might be
other files in the folder than the original files, and the installer
doesn't currently keep a list of installed files.)
distro-build: try connecting a few times to let a VM warm up

This strategy has been successful for the package-build service,
and it will hopefully cut down on spurious snapshot build failures
that happen when a VM's networking doesn't start up quickly enough.