Add check for MACOSX

[carolinan]

Add a new check for the __MACOSX directory.
Level: Required (error).

Partial for #403

Replaces #414 because TeBenachi is not actively contributing right now.

The difference between the two PR's:
Checks the theme root folder of the chosen theme from the plugin settings, not the active theme.
Small adjustment to the error message text.

Testing Instructions

You need at least two themes installed.

1. In the theme that is not active, add a new folder with the name __MACOSX
2. (Or install a theme that has been actually zipped on a Mac: I don't have a Mac, so I cheated)
3. Run the theme check on the theme with the folder, and confirm that the correct error message shows
4. Run the theme check on the theme without the folder and confirm that the error message does not show.

[carolinan]

there are still issues with this, going to try something else.