Show error if no coursebook exists for this user

aleksis/apps/alsijil/templates/alsijil/class_register/select_coursebook.html

@@ -27,6 +27,29 @@
           </v-card-actions>
         </v-card>
       </v-col>
+    {% empty %}
+      <v-container
+        class="text-center fill-height"
+        style="height: calc(100vh - 58px);"
+      >
+        <v-row align="center">
+          <v-col>
+            <h1 class="text-h3 primary--text">
+              <v-icon color="error" x-large>mdi-book-off-outline</v-icon>
+              Whoops, 404
+            </h1>
+
+            <p>The page you were looking for does not exist</p>
+
+            <v-btn
+              color="primary"
+              outlined
+            >
+              Get me out of here!
+            </v-btn>
+          </v-col>
+        </v-row>
+      </v-container>
     {% endfor %}
   </v-row>
 {% endblock %}