How to use the private/core.err function in private

this.includeDependencies = desc.location === 'remote';
    }
    
    if (!includeDependencies) return done(); // skip
    
    // map to array to process in parallel
    var deps = [];
    for(var packageId in desc.dependencies) {
      if (!desc.dependencies.hasOwnProperty(packageId)) continue;
      var version = desc.dependencies[packageId];
      deps.push({ packageId: packageId, version: version });
    }
    
    CORE.iter.parallel(deps, function(dep, done) {
      context.install(dep.packageId, dep.version, false, false, done);
    })(CORE.err(done));
  }

(function(done) {
      if (!CORE.fs.exists(dstPath)) return done();  // nothing to do
      
      if (!force) {
        return done(new Error(packageId+' already exists in working package.  Use --force to override'));
      }

      // delete package if it exists so we can replace it
      CORE.verbose("Removing existing package at "+dstPath);
      CORE.fs.rm_r(dstPath, CORE.err(done));
        
    // if no error, do copy.
    })(function(err) {
      if (err) return done(err);

remoteFor(state.remote, function(err, remote) {
    if (err || !remote) return done(err || '(No Remote)');
    return remote.signup(CORE.err(done));
  });
};

context.prepare(desc, function(err) {
              if (err) return done(err);
              context.installDependencies(desc, CORE.err(done));
            });

CORE.fs.mkdir_p(CORE.path.dirname(dst), 511, function(err) {
            if (err) CORE.println('mkdir_p ' + CORE.path.dirname(dst));
            if (err) return done(err);
            CORE.fs.cp(src, dst, CORE.err(done));
          });

remoteFor(state.remote, function(err, remote) {
    if (err || !remote) return done(err || '(No Remote)');
    return remote.logout(usernames, CORE.err(done));
  });
};

context.source.install(pkg, function(err) {
                if (err) return done(err);
                if (desc.remote) {
                  desc.remote.cleanup(desc.path, CORE.err(done));
                } else return done();
              });
            });

remoteFor(state.remote, function(err, remote) {
    if (err || !remote) return done(err || '(No Remote)');
    return remote.login(usernames, CORE.err(done));
  });
};

})(function(err, workingPackage) {
    if (err) return done(err);
    CORE.verbose("Working package at "+workingPackage.path);
    CORE.iter.each(packageIds, 
      freezePackage(workingPackage, version, force, dirname)
    )(CORE.err(done));
  });
};

})(function(err, workingPackage) {
    if (err) return done(err);
    CORE.verbose("Working package at "+workingPackage.path);
    CORE.iter.each(packageIds, 
      forkPackage(workingPackage, version, force, dirname)
    )(CORE.err(done));
  });
};