Add a better message when an ICS is invalid

Is your feature request related to a problem? Please describe.

Following #8332 , it might be good to get a better message than "there is an error" when trying to import an invalid ICS calendar.

Describe the solution you'd like

Some kind of message like "This calendar is invalid", and it could be even better if that gives a direct link to the validator, like "Please check the validator: https://icalendar.org/validator.html" ; or even better "This calendar is invalid, check it here: https://icalendar.org/validator.html.html?url=https://foo.bar.tld/calendar.ics" , which could give a direct knowledge of the trouble to the user.

Trace the calendar import flow to find where the generic error "there is an error" is produced; the issue does not name a file or test. Review how the imported calendar URL is available before deciding whether to show only an invalid-calendar message or also a validator link. Done means invalid ICS imports give a more useful, user-facing message.