Magma guardrail threshold KV response needs an error message

Raised by MB-59804.

Due to loading data into the bucket until the resident ratio guardrail has been breached,
The resident ratio of the bucket on 2 nodes after data loading is [44.52, 44.18], which is the below the configuredMinimum (50%). Thus, KV mutations are blocked.

If we perform a restore on this bucket using cbbackupmgr, we get the following error message: 
unknown kv status code (54)
Which is triggered by KV here: https://src.couchbase.org/source/xref/trunk/kv_engine/include/mcbp/protocol/status.h?r=67f3ed08#97

Then gocbcore triggers the unknown status code error here: 
https://github.com/couchbase/gocbcore/blob/1cc2ded6f8cc2e1fcccc070b52cad843e94034ea/memd/statuscode.go#L335

We need a constant defined by gocbcore for this error status code.