Add a syncblocked state for cases where a folder sync is blocked because the folder isn't syncable yet

f? @asutherland: I'm not sure what the best way to clear out all the active slices when we get an error from syncFolderList, but otherwise, this should work. This also needs https://github.com/mozilla-b2g/gaia/pull/7409 to work for the UI side of things.

https://bugzilla.mozilla.org/show_bug.cgi?id=827333