Add exit codes

[jo-sm]

This adds support for exit codes, similarly to the stylelint CLI.

The original stylelint CLI supports 4 exit codes:

1: Something unknown went wrong.
2: At least one rule with an "error"-level severity triggered at least one violations.
78: There was some problem with the configuration file.
80: A file glob was passed, but it found no files.

This pull request adds 5 new exit codes, from 1 to 5, and simplifies the exit codes that are present in stylelint:

1. A catch all for all unknown errors that occur within stylelint_d, such as invalid JSON or internal messages being sent from the server instance to the client.
2. When the CSS file contains an error (not just a warning).
3. The configuration file has an issue (stylelint code 78).
4. A file glob was passed but had no files (stylelint code 80).
5. A catch all for other error codes sent from stylelint itself.

Thanks to @Mukimov for this idea!

[mufasa71]

@jo-sm Thanks, looks great!