Improve UX in landscape config dialogs

Improve validation and cancel button behavior in landscape config by using return value of whiptail to determine cancel vs. empty string, and adding validation error messages.

Requires a duplicate version of dialogInput that doesn't always return true, so we can check whiptail's return value. Thus, we also need to set +e around the usage of whiptail.

This is a little hacky, hence the pull request for comments.