processes: Don't return error if process exited

[PierreF]

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I've sometime the following error:

$ grep telegraf /var/log/syslog |grep Error
May 26 06:28:20 hostname telegraf[15844]: 2016/05/26 06:28:20 Error in input [processes]: open /proc/16683/stat: no such file or directory
May 26 07:21:30 hostname telegraf[15844]: 2016/05/26 07:21:30 Error in input [processes]: read /proc/19975/stat: no such process
May 26 09:29:40 hostname telegraf[15844]: 2016/05/26 09:29:40 Error in input [processes]: read /proc/27969/stat: no such process
May 26 09:40:40 hostname telegraf[15844]: 2016/05/26 09:40:40 Error in input [processes]: read /proc/28637/stat: no such process

This cause the processes plugin to skip all metrics for the affected collection round.

This PR add a second file.IsDir() during error handling, if path is no longer a directory then the process must have terminated so it continues instead of returning an error

[sparrc]

thx @PierreF