get_meeting_data() doesn't handle errors

[tellyworth]

get_meeting_data() assumes that the REST server returns some meetings, and json encodes its response. This results in an internal error message in the data-meetings attribute, and JS console errors on the front end, with no visible content.

Instead, get_meeting_data() should check for an error - I think is_wp_error() will catch it - and render_callback() should probably display a meaningful error.