Handle invalid calendar id gracefully

[charlesbrandt]

We're finding that users are confused about what value to enter in the calendar ID field. Often they will try a url copied from google calendar. This results in an unhandled exception that locks them out of the page.

This is fixed by editing the page and removing the invalid value.

Ideally the module should catch this exception and report an invalid ID was entered. There could even be a link to the main readme for instructions on finding a valid Google Calendar ID:

https://github.com/City-of-Bloomington/drupal-module-calendar

[inghamn]

We need to write a custom field validation for the calendar module.

https://www.drupal.org/docs/drupal-apis/entity-api/entity-validation-api/providing-a-custom-validation-constraint