Improve "empty timezone" handling in the schedule edition

[viroulep]

Similarly to #4638, when submitting a schedule with an empty timezone the user get an error message about the timezone, but it's not the most user friendly. We could use the same trick as in #4978 and lower the error detection down to the model, to be able to emit a custom message.

Additionally, creating a custom activity results in a javascript error if the timezone is undefined. One way to fix this would be to replace the calendar by an information message if the timezone for the venue is undefined.

[jpbulman]

I think I'm going to try and take a look at this, if nobody else has