Field errors are persisted in between calls

[splittingred]

Have been able to confirm this by creating an RPC method that adds field errors depending on incoming value:

context 'with multiple calls, where the first returns an error, the second doesnt' do
  it 'should not raise a Gruf::Client::Error the second time', run_thing_server: true do
    client = build_client
    expect do
      client.call(:GetContextualFieldErrorFail, id: 1)
    end.to raise_error(Gruf::Client::Error)

    expect do
      client.call(:GetContextualFieldErrorFail, id: 2)
    end.to_not raise_error(Gruf::Client::Error)
  end
end

It will throw an error currently:

expected no Gruf::Client::Error, got #<Gruf::Client::Error: Gruf::Client::Error> with backtrace:

This seems to be due to @error being set memoized on the service class, and not reset in-between calls. This was designed in order to support the add_field_error method without having to pass around an error object. A simple fix is just to unset the @error attribute at the end of every call.