Remove logentries-system-stats file creation,add try catch when accessing stats

This remove the logentries-system-stats.log file creation in the install script.

Also added a try catch for when we try to access the system metrics dictionary in case such values don't exist ([See SmartOS issue](https://github.com/logentries/le/issues/60)). For the moment if there is an exception when trying to access one of these objects then the stats will not be sent.