Migrate Get/GetFromMaster methods from ChannelStore to return error interface (#14688)

* Advances

* Migration finished

* Rename err to normalized error

* fix imports

* Renamed key

* Renamed key

* Suggestions

* Fix i18n

* Fix tests

Co-authored-by: Jesús Espino <jespinog@gmail.com>
Co-authored-by: Agniva De Sarker <agnivade@yahoo.co.in>

app/post.go

@@ -5,6 +5,7 @@ package app
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net/http"
 	"strings"
@@ -94,7 +95,13 @@ func (a *App) CreatePostAsUser(post *model.Post, currentSessionId string, setOnl
 func (a *App) CreatePostMissingChannel(post *model.Post, triggerWebhooks bool) (*model.Post, *model.AppError) {
 	channel, err := a.Srv().Store.Channel().Get(post.ChannelId, true)
 	if err != nil {
-		return nil, err
+		var nfErr *store.ErrNotFound
+		switch {
+		case errors.As(err, &nfErr):
+			return nil, model.NewAppError("CreatePostMissingChannel", "app.channel.get.existing.app_error", nil, nfErr.Error(), http.StatusNotFound)
+		default:
+			return nil, model.NewAppError("CreatePostMissingChannel", "app.channel.get.find.app_error", nil, err.Error(), http.StatusInternalServerError)
+		}
 	}
 
 	return a.CreatePost(post, channel, triggerWebhooks, true)