MagmaKVStore incorrectly returns no_such_key for all errors

MagmaKVStore::getMulti uses Magma::GetDocs to answer GET queries:

https://github.com/couchbase/kv_engine/blob/3d8ed5630db8138c320971a1d96cbffffd7b59f4/engines/ep/src/kvstore/magma-kvstore/magma-kvstore.cc#L1100

It uses magmaErr2EnginerErr for error handling. Current implementation is:

cb::engine_errc MagmaKVStore::magmaErr2EngineErr(Status::Code err, bool found) {

    if (!found) {

        return cb::engine_errc::no_such_key;

    }

    // This routine is intended to mimic couchErr2EngineErr.

    // Since magma doesn't have a memory allocation error, all magma errors

    // get translated into cb::engine_errc::temporary_failure.

    if (err == Status::Code::Ok) {

        return cb::engine_errc::success;

    }

    return cb::engine_errc::temporary_failure;

} 

It always returns no_such_key when found=false. But found will always be false in presence of errors. We shouldn't  return no_such_key in all such cases. That would be alarming and falsely indicate data loss.

The error handling needs to be granular wrt to the Status::Code. For example a IOError is temporary and hence we should return temporary_failure.