(SERVER-61) Add STDERR handling to execution stub.

This will result in an Exception if the command fails.
For parity with the Ruby Puppet master, should we have a block in the calling `initialize_execution_stub` from Puppet Server's "master.rb" that catches these Exceptions and re-raises with a `Puppet::ExecutionFailure` wrapper?  Like this:

https://github.com/puppetlabs/puppet/blob/master/lib/puppet/util/execution.rb#L104

Not sure if there's something higher up in the call stack that would look for this kind of exception in particular and do something if it is thrown...