Fix RemoteCalendarConnectionError init

[rwood-moz]

When a RemoteCalendarConnectionError() is raised and no reason is provided the result is a TypeErrror:

>               raise RemoteCalendarConnectionError()
E               TypeError: RemoteCalendarConnectionError.__init__() missing 1 required positional argument: 'reason'

There are several existing cases of RemoteCalendarConnectionError() being used with no reason value provided.

The fix is to have the RemoteCalendarConnectionError() reason parameter to have a default value of None.

[devmount]

Good catch, thank you!