[r=dimitern] state;api: Allow adding existing networks/NICs
Added an AlreadyExistsError in errors/ and in
state/api/params with IsAlreadyExistsError and
IsCodeAlreadyExists helpers. This is used in
the provisioner API to report networks and
interfaces that already exist, and is needed
so that when provisioning more machines on the
same network wont' lead to errors (trying to
add existing network/NIC is just ignored).
https://codereview.appspot.com/85380043/