[core] Fixing bug in setField. Standard messages for readonly.
[core] Fixing bug in setField. Standard messages for readonly.

openconnectome/open-connectome,neurodata/ndstore,openconnectome/open-connectome,openconnectome/open-connectome,openconnectome/open-connectome,neurodata/ndstore,neurodata/ndstore,openconnectome/open-connectome,neurodata/ndstore,openconnectome/open-connectome