Change NFSMount._availability_errors to a property
Change NFSMount._availability_errors to a property

All availability errors are properties, not methods (including
superclass of NFSMount).

vojtechtrefny/blivet,jkonecny12/blivet,rvykydal/blivet,jkonecny12/blivet,rvykydal/blivet,vojtechtrefny/blivet