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>

api4/post_test.go

@@ -871,7 +871,7 @@ func TestPatchPost(t *testing.T) {
 
 	t.Run("invalid requests", func(t *testing.T) {
 		r, err := Client.DoApiPut("/posts/"+post.Id+"/patch", "garbage")
-		require.EqualError(t, err, ": Invalid or missing post in request body, ")
+		require.EqualError(t, err, ": Invalid or missing post in request body., ")
 		require.Equal(t, http.StatusBadRequest, r.StatusCode, "wrong status code")
 
 		patch := &model.PostPatch{}