silently ignore a bad/missing --defaults-torrc

The nightly Debian packages for wheezy were broken because they were missing /usr/share/tor/tor-service-defaults-torrc. The issue was not easy to diagnose because Tor currently silently ignore a --defaults-torrc pointing to a non-existing file.

At least a warning would be good, but I think it would be even better to bail out unless --ignore-missing-torrc is specified.