Simplify alert messaging

- decouple messaging from error
- update tests

app/assets/javascripts/pages/shared/wikis/components/wiki_alert.vue

 <script>
 import { GlAlert, GlLink, GlSprintf } from '@gitlab/ui';
-import { ERRORS } from '../constants';
 
 export default {
-  ERRORS,
   components: {
     GlAlert,
     GlLink,
@@ -16,18 +14,7 @@ export default {
     },
     wikiPagePath: {
       type: String,
-      required: false,
-      default: '',
-    },
-  },
-  computed: {
-    errorMessage() {
-      if (this.error === this.$options.ERRORS.PAGE_CHANGE.ERROR) {
-        return this.$options.ERRORS.PAGE_CHANGE.MESSAGE;
-      } else if (this.error === this.$options.ERRORS.PAGE_RENAME.ERROR) {
-        return this.$options.ERRORS.PAGE_RENAME.MESSAGE;
-      }
-      return this.error;
+      required: true,
     },
   },
 };
@@ -35,7 +22,7 @@ export default {
 
 <template>
   <gl-alert variant="danger" :dismissible="false">
-    <gl-sprintf :message="errorMessage">
+    <gl-sprintf :message="error">
       <template #wikiLink="{ content }">
         <gl-link :href="wikiPagePath" target="_blank">{{ content }}</gl-link>
       </template>

app/assets/javascripts/pages/shared/wikis/constants.js

-import { s__ } from '~/locale';
-
-export const ERRORS = {
-  PAGE_CHANGE: {
-    ERROR: 'WikiPage::PageChangedError',
-    MESSAGE: s__(
-      'WikiPageConflictMessage|Someone edited the page the same time you did. Please check out %{wikiLinkStart}the page%{wikiLinkEnd} and make sure your changes will not unintentionally remove theirs.',
-    ),
-  },
-  PAGE_RENAME: {
-    ERROR: 'WikiPage::PageRenameError',
-    MESSAGE: s__('WikiEdit|There is already a page with the same title in that path.'),
-  },
-};

app/controllers/concerns/wiki_actions.rb

@@ -116,7 +116,7 @@ module WikiActions
       render 'shared/wikis/edit'
     end
   rescue WikiPage::PageChangedError, WikiPage::PageRenameError => e
-    @error = e
+    @error = e.message
     render 'shared/wikis/edit'
   end
   # rubocop:enable Gitlab/ModuleWithInstanceVariables

app/models/wiki_page.rb

@@ -205,14 +205,15 @@ class WikiPage
     last_commit_sha = attrs.delete(:last_commit_sha)
 
     if last_commit_sha && last_commit_sha != self.last_commit_sha
-      raise PageChangedError
+      raise PageChangedError, s_(
+        'WikiPageConflictMessage|Someone edited the page the same time you did. Please check out %{wikiLinkStart}the page%{wikiLinkEnd} and make sure your changes will not unintentionally remove theirs.')
     end
 
     update_attributes(attrs)
 
     if title.present? && title_changed? && wiki.find_page(title).present?
       attributes[:title] = page.title
-      raise PageRenameError
+      raise PageRenameError, s_('WikiEdit|There is already a page with the same title in that path.')
     end
 
     save do

app/views/shared/wikis/edit.html.haml

 - wiki_page_title @page, @page.persisted? ? _('Edit') : _('New')
 - add_page_specific_style 'page_bundles/wiki'
-- errorMessage = @error || ''
 
 - if @error
-  #js-wiki-error{ data: { error: errorMessage, wiki_page_path: wiki_page_path(@wiki, @page) } }
+  #js-wiki-error{ data: { error: @error, wiki_page_path: wiki_page_path(@wiki, @page) } }
 
 .wiki-page-header.top-area.has-sidebar-toggle.flex-column.flex-lg-row
   = wiki_sidebar_toggle_button

spec/frontend/pages/shared/wikis/wiki_alert_spec.js

 import { GlAlert, GlLink, GlSprintf } from '@gitlab/ui';
 import { shallowMount } from '@vue/test-utils';
 import WikiAlert from '~/pages/shared/wikis/components/wiki_alert.vue';
-import { ERRORS } from '~/pages/shared/wikis/constants';
 
 describe('WikiAlert', () => {
   let wrapper;
+  const ERROR = 'There is already a page with the same title in that path.';
+  const ERROR_WITH_LINK =
+    'Someone edited the page the same time you did. Please check out %{wikiLinkStart}the page%{wikiLinkEnd} and make sure your changes will not unintentionally remove theirs.';
+  const PATH = '/test';
 
   function createWrapper(propsData = {}, stubs = {}) {
     wrapper = shallowMount(WikiAlert, {
-      propsData,
+      propsData: { wikiPagePath: PATH, ...propsData },
       stubs,
     });
   }
@@ -24,32 +27,15 @@ describe('WikiAlert', () => {
 
   describe('Wiki Alert', () => {
     it('does show an alert when there is an error', () => {
-      createWrapper({ error: ERRORS.PAGE_RENAME.ERROR });
+      createWrapper({ error: ERROR });
       expect(findGlAlert().exists()).toBe(true);
-    });
-
-    it('does show the page change message text', () => {
-      createWrapper({ error: ERRORS.PAGE_CHANGE.ERROR });
       expect(findGlSprintf().exists()).toBe(true);
-      const text = findGlSprintf();
-      expect(text.attributes('message')).toBe(ERRORS.PAGE_CHANGE.MESSAGE);
-    });
-
-    it('does show the page rename message text', () => {
-      createWrapper({ error: ERRORS.PAGE_RENAME.ERROR });
-      expect(findGlSprintf().attributes('message')).toBe(ERRORS.PAGE_RENAME.MESSAGE);
-    });
-
-    it('does show the error message', () => {
-      const error = 'test message';
-      createWrapper({ error });
-      expect(findGlSprintf().attributes('message')).toBe(error);
+      expect(findGlSprintf().attributes('message')).toBe(ERROR);
     });
 
     it('does show the link to the help path', () => {
-      const wikiPagePath = '/help';
-      createWrapper({ error: ERRORS.PAGE_CHANGE.ERROR, wikiPagePath }, { GlAlert, GlSprintf });
-      expect(findGlLink().attributes('href')).toBe(wikiPagePath);
+      createWrapper({ error: ERROR_WITH_LINK }, { GlAlert, GlSprintf });
+      expect(findGlLink().attributes('href')).toBe(PATH);
     });
   });
 });