Added error message and test

app/views/projects/mattermosts/_no_teams.html.haml

+= content_for :flash_message do
+  .alert.alert-danger= @teams if @teams.is_a?(String)
+
 %p
   You aren’t a member of any team on the Mattermost instance at
   %strong= Gitlab.config.mattermost.host

app/views/projects/mattermosts/new.html.haml

@@ -2,7 +2,7 @@
   .inline.pull-right
     = custom_icon('mattermost_logo', size: 48)
   %h3 Install Mattermost Command
-  - if @teams.empty?
+  - if @teams.is_a?(String) || @teams.empty?
     = render 'no_teams'
   - else
     = render 'team_selection'

spec/features/projects/services/mattermost_slash_command_spec.rb

@@ -99,6 +99,15 @@ feature 'Setup Mattermost slash commands', feature: true do
       expect(select_element.all('option').count).to eq(3)
     end
 
+    it 'shows an error alert with the error message if there is an error requesting teams' do
+      allow_any_instance_of(MattermostSlashCommandsService).to receive(:list_teams) { 'test mattermost error message' }
+
+      click_link 'Add to Mattermost'
+
+      expect(page).to have_selector('.alert')
+      expect(page).to have_content('test mattermost error message')
+    end
+
     def stub_teams(count: 0)
       teams = create_teams(count)