Handle unexpected return codes

Previously, these would throw a TypeError because of an attempt to raise
nil, which was a little confusing. This implementation should make them
easier to track.

lib/memcached/memcached.rb

@@ -610,7 +610,11 @@ Please note that when <tt>:no_block => true</tt>, update methods do not raise on
         end
       end
     end
-    raise EXCEPTIONS[ret], message
+    if EXCEPTIONS[ret]
+      raise EXCEPTIONS[ret], message
+    else
+      raise Memcached::Error, "Unknown return code: #{ret}"
+    end
   end
 
   # Turn an array of keys into a hash of keys to servers.

test/unit/memcached_test.rb

@@ -1279,6 +1279,14 @@ class MemcachedTest < Test::Unit::TestCase
       @cache.instance_variable_get("@struct")
   end
 
+  # NOTE: This breaks encapsulation, but there's no other easy way to test this without
+  # mocking out Rlibmemcached calls
+  def test_reraise_invalid_return_code
+    assert_raise Memcached::Error do
+      @cache.send(:check_return_code, 5000)
+    end
+  end
+
   private
 
   def key