Fix issue FACT-715: LXC detection code raises errors on old OpenVZ.

This patch fixes issue FACT-715. It works for me and it should not break anything.
Is there a reason for the string comparison here?  If the exception was thrown, `in_lxc` will be `nil`, resulting in false anyway.  So from what I can see, returning false in the rescue regardless of message is the correct course of action.