Improve error handling when pasting an invalid link as camp template

We should notify the user if their URL is invalid.
Currently it says its required, even if the user provided a value. Rather than the generic required message, it should display a message that the url is not valid. Also after clicking the button, the user expects the textfield next to it to be filled with the value from the clipboard (even if invalid).

_Originally posted by @manuelmeister in https://github.com/ecamp/ecamp3/pull/7528#pullrequestreview-3044301291_