Handle failure of conference loading

[tiainen]

The conferences are now loaded by invoking a CloudLink remote function instead of parsing the data locally from a JSON file. It is a good idea to handle any failures that occur while invoking the remote function.

Some improvements that can be implemented:

• add a notification message informing the user that loading conferences did not succeed
• add a reload button to reload the list of conferences
• an automatic reload process that automatically invokes the remote function again with increasingly longer intervals in case of failures