Puppet::Agent::Locker#lock doesn't return whether it acquired the lock or not

The puppet agent uses a lock file to ensure that only one instance is running at a time. However, the agent was susceptible to a race condition that could cause two agents to try to acquire the lock at the same time, and have one of them fail with a generic "Could not run" error. Now the agent will atomically try to acquire the lock, and if that fails, log a meaningful error.