Pony should check that paths for hostname requests exist

[achernya]

Our current policy is to require a path exists before an MIT hostname is assigned. Pony should check and if the path exists, and require the user fix the path before proceeding. This will help prevent typos.

Pony should also offer a link to the administrators to check if the site actually exists, because then users will just create blank index.html files to get around the requirement.