MM-24132: Migrate AppError from SaveDirectChannel/channel_store.go (#14318)

* MM-24135: Migrate AppError from SaveChannel/channel_store.go

This is the first POC of migration of store app errors to plain error.

We create a few basic error types in the store package and use
them to return the errors from store methods. In the app layer,
we inspect the error and re-create the exact app errors. This lets
us preserve the same error content, but yet move to plain errors.

Since this is a gradual migration, this means that the error inspection
code will be duplicated across the app layer whenever a store method
is invoked. But all of that should go away once we start propagating
the errors higher up the hierarchy.

There have been a significant amount of changes in the storetest and searchtest
layer, primarily because we have to rename the err variable now that it is of
a different type.

* Addressed review comments

* MM-24132: Migrate AppError from SaveDirectChannel/channel_store.go

This PR migrates 2 new methods SaveDirectChannel and CreateDirectChannel
to return error instead of AppError.

We also need to handle the error internally in SaveMultipleMember for now
until that is migrated too.

* Fix layers

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

store/searchlayer/channel_layer.go

@@ -97,7 +97,7 @@ func (c *SearchChannelStore) RemoveMember(channelId, userIdToRemove string) *mod
 	return err
 }
 
-func (c *SearchChannelStore) CreateDirectChannel(user *model.User, otherUser *model.User) (*model.Channel, *model.AppError) {
+func (c *SearchChannelStore) CreateDirectChannel(user *model.User, otherUser *model.User) (*model.Channel, error) {
 	channel, err := c.ChannelStore.CreateDirectChannel(user, otherUser)
 	if err == nil {
 		c.rootStore.indexUserFromID(user.Id)