fix error displays on apps form; improve connection checking

to test:

1) add app with a non-working URL. Upon submit, there should only be an error icon and error message below the URL field, instead of an error icon below every field.
2) on your machine, make sure nothing is listening on port 80 or port 443. Then run `sudo nc -l -p 443` on your host machine. Then add an app with the URL `https://<your client ip address>` (substituting your ip address). The submission should succeed.