Clearer error message if using serve in the wrong dir

[tlienart]

See #154

If using serve in the wrong dir something like:

julia> serve()
ERROR: SystemError: unable to read directory /Users/tlienart/Desktop/tjd/src: No such file or directory

will pop up, would be better to have a clean error indicating that no src/ folder has been found and therefore that the user is probably not in the right spot.