Using unregistered Timezones should throw a warning

[CardiJey]

Hi,
I've been wondering for a long time why something goes wrong with the timezones when using Time.toJSDate().
Only now, I found out, that I did not register the timezone of the ical file beforehand. I think you can save a lot of people a lot of time if the function throws some kind of error when using an unregistered timezone, instead of just defaulting to UTC.

[kewisch]

This sounds reasonable, though we'll have to find out in which cases to throw and where. For anyone looking into this, keep in mind that the IANA timezones, such as Europe/Berlin, are valid even if not explicitly in a VTIMEZONE block.