Allow proxy to pass NBD protocol errors downstream; server returns EINVAL/ENOSPC appropriately

Previously the proxy would just disconnect when it saw an NBD protocol
error, and retry the operation it was in the middle of.

Additionally, the server needs to return the correct error types when
this happens.

tests/acceptance/proxy_tests.rb

@@ -72,6 +72,20 @@ module ProxyTests
     end
   end
 
+  def test_write_request_past_end_of_disc_returns_to_client
+    with_proxied_client do |client|
+      n = 1000
+      offset = n * 4096
+      client.write(offset, b * 4096)
+      rsp = client.read_response
+
+      assert_equal FlexNBD::REPLY_MAGIC, rsp[:magic]
+      assert_equal 'myhandle', rsp[:handle]
+      # NBD protocol say ENOSPC (28) in this situation
+      assert_equal 28, rsp[:error]
+    end
+  end
+
   def make_fake_server
     server = FlexNBD::FakeDest.new(@env.ip, @env.port1)
     @server_up = true