dgram: make addMembership() and dropMembership() conform to v0.4 API

- throw on error, don't return an error code

lib/dgram.js

@@ -260,7 +260,9 @@ Socket.prototype.addMembership = function(multicastAddress,
     throw new Error('multicast address must be specified');
   }
 
-  return this._handle.addMembership(multicastAddress, interfaceAddress);
+  if (this._handle.addMembership(multicastAddress, interfaceAddress)) {
+    throw new errnoException(errno, 'addMembership');
+  }
 };
 
 
@@ -272,7 +274,9 @@ Socket.prototype.dropMembership = function(multicastAddress,
     throw new Error('multicast address must be specified');
   }
 
-  return this._handle.dropMembership(multicastAddress, interfaceAddress);
+  if (this._handle.dropMembership(multicastAddress, interfaceAddress)) {
+    throw new errnoException(errno, 'dropMembership');
+  }
 };