Use flash-error instead of error where appropriate

It's nicer :)

owl/templates/join-token.html

@@ -4,7 +4,7 @@
 	<h2>Join a new course{% if course %}: {{ course.name }}{% endif %}</h2>
 
 	{% if error %}
-		<p class=error>{{ error }}</p>
+		<p class="flash flash-error">{{ error }}
 		<div class=buttons>
 			<a href='{{ url_for('index') }}' class=button>Go to main page</a>
 		</div>

owl/templates/join.html

@@ -4,7 +4,7 @@
 	<h2>Join a new course</h2>
 
 	{% if error %}
-	<p class=error>{{ error }}
+		<p class="flash flash-error">{{ error }}
 	{% else %}
 		<p>To join a course, you need to enter a code distributed by your teacher.
 	{% endif %}

owl/templates/login.html

@@ -10,7 +10,7 @@
 	</form>
 
 	{% if error %}
-	<p class=error>{{ error }}
+	<p class="flash flash-error">{{ error }}
 	{% endif %}
 
 	<form method="POST" action="?">