Handle the case of existing CSS module files

[umaar]

When we detect an existing theme file is present, we do not overwrite it, however this same behavior doesn't kick in for existing CSS module files meaning they can get overwritten. Ideally we would inform the user of:

• The path of the existing CSS module file
• The fact that we will not overwrite it
• The list of CSS selectors which they can manually merge into their CSS file if they wish to

[KaneFreeman]

We are addressing #9 by simply throwing an error prior to the process starting. Should we do the same here or treat these differently from the main theme file?