clarified error message.

scss-lint exit code 66 is used not only when a file doesn't exist or is not
readable, but also when no files were specified or all specified files
were also excluded by the --exclude option.
we don't actually care about the file contents since only the file paths
are passed to the scss-lint binary.
the plugin should not error if there are no files.

scss-lint itself will return an error if it is not given any files.
That's not something we consider an error condition, so do not run
scss-lint if no files have been provided to the stream.
the plugin should error if a non-null file is provided with no path.

Since we're passing file paths to scss-lint it should be an error to ask
the plugin to lint a file with no path.