better document #del method

Thanks for the feedback, @rvagg.
Based on your feedback and some double checking via
https://github.com/rvagg/node-leveldown/blob/5d552d750170bedd0656d2e6d82673556fa0ea0d/deps/leveldb/leveldb-1.17.0/db/db_impl.cc#L1437-L1441

I've gone with a comment similar to the preceding stanza.
Also removed the curly braces.

README.md

@@ -258,9 +258,9 @@ LevelDB will by default fill the in-memory LRU Cache with data from a call to ge
 ### db.del(key[, options][, callback])
 <code>del()</code> is the primary method for removing data from the store.
 ```js
-db.del('foo', function hasError (err) {
-  console.error('ERROR', err);
-  throw err;
+db.del('foo', function (err) {
+  if (err)
+    // handle I/O or other error
 });
 ```