Attach connection errors to socket

Raising the errors would crash the current thread. As this thread
contains the Eventmachine, that would influence other connections
as well. Attaching the errors to the connection and reading them
after the connection was closed ensures that the thread stays
alive while handling the errors in the main thread of the current
request.

lib/runner/strategy/poseidon.rb

@@ -89,6 +89,7 @@ class Runner::Strategy::Poseidon < Runner::Strategy
     websocket_url = execute_command(command)
     socket = Connection.new(websocket_url, self, event_loop)
     yield(socket)
+    socket
   end
 
   def destroy_at_management
@@ -130,7 +131,8 @@ class Runner::Strategy::Poseidon < Runner::Strategy
     def decode(raw_event)
       JSON.parse(raw_event.data)
     rescue JSON::ParserError => e
-      raise Runner::Error::UnexpectedResponse.new("The WebSocket message from Poseidon could not be decoded to JSON: #{e.inspect}")
+      @error = Runner::Error::UnexpectedResponse.new("The WebSocket message from Poseidon could not be decoded to JSON: #{e.inspect}")
+      close(:error)
     end
 
     def encode(data)