Show error message when configured default tool is not installed

Closes PLAT-4804


Test Plan:
- configure a default tool that is not installed
- verify an error is dispalyed when creating an assignment

Change-Id: I90b395c441cd07e884d1b71244a95bf72dc03020
Reviewed-on: https://gerrit.instructure.com/205866
Tested-by: Jenkins
Reviewed-by: Xander Moffatt <xmoffatt@instructure.com>
QA-Review: Xander Moffatt <xmoffatt@instructure.com>
Product-Review: Weston Dransfield <wdransfield@instructure.com>

app/jsx/assignments/DefaultToolForm.js

@@ -68,6 +68,17 @@ const DefaultToolForm = props => {
     SelectContentDialog.Events.onContextExternalToolSelect(event, $('#default-tool'))
   }
 
+  if(!defaultToolData && launchDefinitions.length > 0) {
+    return(
+      <View display="block" padding="medium none small small">
+        <Alert variant="error" margin="small small 0 0">
+          <Text weight="bold">{I18n.t('Tool Not Found')}</Text><br/>
+          <Text>{I18n.t('The tool is not installed in the course or account')}</Text>
+        </Alert>
+      </View>
+    )
+  }
+
   return (
     <View display="block" padding="medium none small small">
       <Button id="default-tool-launch-button" name="default-tool-launch-button" onClick={handleLaunchButton}>

app/jsx/assignments/__tests__/DefaultToolForm.test.js

@@ -16,6 +16,7 @@
  * with this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
+import axios from 'axios'
 import React from 'react'
 import {mount} from 'enzyme'
 import DefaultToolForm, { toolSubmissionType } from '../DefaultToolForm'
@@ -31,6 +32,17 @@ const newProps = (overrides = {}) => ({
   ...overrides
 })
 
+beforeAll(() => {
+  axios.get = jest.fn()
+  axios.get.mockImplementation(async () => {
+    return {data: []};
+  })
+})
+
+afterEach(() => {
+  axios.get.mockRestore()
+})
+
 describe('DefaultToolForm', () => {
   let wrapper = 'empty wrapper'
 
@@ -65,6 +77,23 @@ describe('DefaultToolForm', () => {
     wrapper = mount(<DefaultToolForm {...newProps({previouslySelected: true})} />)
     expect(wrapper.find('Alert').html()).toContain('Successfully Added')
   })
+
+  describe('when the configured tool is not installed', () => {
+    beforeAll(() => {
+      axios.get.mockImplementation(async () => {
+        return {data: [{
+          placements: [{
+            url: 'foo'
+          }]
+        }]};
+      })
+    })
+
+    it('renders an error message', async () => {
+      wrapper = mount(<DefaultToolForm {...newProps({previouslySelected: true})} />)
+      expect(wrapper.find('Alert').exists()).toEqual(true)
+    })
+  })
 })
 
 describe('toolSubmissionType', () => {