Rails show error message of another model - ruby-on-rails

Question

I have a html partial that displays errors:
shared/model_errors.html.erb
<% model.errors.full_messages.each do |msg| %>
and I render it at top of each html file like
<%= render partial: 'shared/model_errors',locals: {model: #model_to_check_errors}%>
This is working fine for most of them.
Let's say that I'm on the show.html.erb template of the ConversationsController, and I have there a form_tag which submits messages to the MessagesController. If the submitted message is empty, the MessagesController won't allow it to be saved, because it has validation errors. How do I display those validations errors in my show.html.erb, considering I don't have access to the #message object of the create action of the MessagesController. Is there any way to add the error message to the ConversationsController? They will be displayed that way.
Edit:
show.html.erb of the ConversationsController
<div class="container nav_exp">
<%= render partial: 'shared/model_errors',locals: {model: #???}%>
<div class="ch-body">
<div class="col-md-12 bg-white ">
<div class="chat-message">
<ul class="chat" id="messages">
<% if #messages != nil %>
<%= render partial: 'conv_message', collection: #messages,:as => :message%>
<% else %>
<div class="panel-body hidden"></div>
<%end%>
</ul>
</div>
<%=form_tag(messages_path, :method=>'post',authenticity_token: true,remote: true) do%>
<div class="chat-box bg-white">
<div class="input-group">
<%= text_field_tag :body,"", id:"message_body",class: "form-control border no-shadow no-rounded", placeholder: "Type your message here"%>
<%= hidden_field_tag :conversation_id, #conversation.id %>
<%= hidden_field_tag :receiver, #other.id %>
<span class="input-group-btn">
<%= submit_tag 'Submit', class: "btn btn-success no-rounded" %>
</span>
</div><!-- /input-group -->
</div>
<% end %>
</div>
</div>
</div>
The create method of the MessagesController - messages_path
def create
message = Message.new(message_params)
message.user = current_user
if message.save
...
else
puts 'not working'
'I think 1 solution would be to add here flash messsages'
end
end
private
def message_params
params.permit(:body, :conversation_id, :receiver)
end