If bad doc root, show link to Settings

[mattpass]

If the user has a doc root set that isn't a valid path, instead of showing nothing in the file manager to the left, show "Select a path..." and a link to Settings > General to set the document root. Could also highlight the input field in Settings modal with a red border to indicate the problem.

[mattpass]

Now showing the bad dir name highlighted in red, with tooltip explaining it's not a dir, and is clickable to open the Settings > General tab.