This PR adds the rather basic functionality of collecting any errors observed in Magic-Folder status API messages and propagating them to the user (via the folders view's "Status" column) like so:

Unfortunately, because Magic-Folder presently provides no API for clearing/acknowledging error messages, until further notice, any/all known Magic-Folder errors will persist indefinitely in the Gridsync UI. Expect this to change/improve as Magic-Folder development continues, however...