Use semantic exit codes for common errors

A commit introduced by #49 pointed out the concept of semantic exit
codes.

This commmit goes a step further by implementing semantic exit codes for
all common sources of errors in `scss-lint`.

For documentation on the various codes, see `/usr/include/sysexits.h`.

Change-Id: I275b90ce3ece7daae8feb47f6b83e277e8630e42
Reviewed-on: https://gerrit.causes.com/31575
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>