Swap 'old' and 'previous' when reporting append error (#4130)
Swap 'old' and 'previous' when reporting append error (#4130)

Fixes #3553
ContinuumIO/dask,mrocklin/dask,jcrist/dask,jcrist/dask,dask/dask,jakirkham/dask,dask/dask,mrocklin/dask,blaze/dask,ContinuumIO/dask,blaze/dask,jakirkham/dask