add login failure warning

UI/main.py

@@ -83,7 +83,7 @@ def login():
         else:
             error = 'An unknown error occurred.'
         
-        flash(error, 'error')
+        flash(error, 'danger')
         return redirect(url_for('login'))
     
     return render_template('login.html', form=form)

UI/templates/login.html

@@ -6,6 +6,21 @@
 <div class="row justify-content-center">
     <div class="col-md-6">
         <h2 class="text-center">Login</h2>
+        
+        <!-- Flash messages -->
+        {% with messages = get_flashed_messages(with_categories=true) %}
+            {% if messages %}
+                {% for category, message in messages %}
+                    <div class="alert alert-{{ category }} alert-dismissible fade show" role="alert">
+                        {{ message }}
+                        <button type="button" class="close" data-dismiss="alert" aria-label="Close">
+                            <span aria-hidden="true">&times;</span>
+                        </button>
+                    </div>
+                {% endfor %}
+            {% endif %}
+        {% endwith %}
+        
         <form id="loginForm" class="needs-validation" novalidate method="POST" action="/login">
             {{ form.hidden_tag() }}
             <div class="form-group">