Fix app error messages (#13852)

* Summary
Fixing inconsistencies of the app_error messages. Now all end with a 
period and there is the same user experience all over the system. 

We agreed on this change on the community server a while ago and I just 
came around finally doing it.


Ticket Link
N/A

Checklist
 Includes text changes and localization file (.../i18n/en.json and 
.../webapp/i18n/en.json) updates

* Update i18n/en.json

Approving changes

Co-Authored-By: Jason Blais <13119842+jasonblais@users.noreply.github.com>

* Update i18n/en.json

Approving changes

Co-Authored-By: Jason Blais <13119842+jasonblais@users.noreply.github.com>

* Update i18n/en.json

Approving changes

Co-Authored-By: Jason Blais <13119842+jasonblais@users.noreply.github.com>

* Fix test failures due string differences

Co-authored-by: Jason Blais <13119842+jasonblais@users.noreply.github.com>
Co-authored-by: Miguel de la Cruz <mgdelacroix@gmail.com>
Co-authored-by: mattermod <mattermod@users.noreply.github.com>

app/notification_push_test.go

@@ -981,7 +981,7 @@ func TestSendPushNotifications(t *testing.T) {
 	t.Run("should return error if data is not valid or nil", func(t *testing.T) {
 		err := th.App.sendPushNotificationToAllSessions(nil, th.BasicUser.Id, "")
 		assert.NotNil(t, err)
-		assert.Equal(t, "pushNotification: An error occurred building the push notification message, ", err.Error())
+		assert.Equal(t, "pushNotification: An error occurred building the push notification message., ", err.Error())
 		// Errors derived of using an empty object are handled internally through the notifications log
 		err = th.App.sendPushNotificationToAllSessions(&model.PushNotification{}, th.BasicUser.Id, "")
 		assert.Nil(t, err)