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/timer_layer.go

@@ -568,7 +568,7 @@ func (s *TimerLayerChannelStore) CountPostsAfter(channelId string, timestamp int
 	return resultVar0, resultVar1
 }
 
-func (s *TimerLayerChannelStore) CreateDirectChannel(userId *model.User, otherUserId *model.User) (*model.Channel, *model.AppError) {
+func (s *TimerLayerChannelStore) CreateDirectChannel(userId *model.User, otherUserId *model.User) (*model.Channel, error) {
 	start := timemodule.Now()
 
 	resultVar0, resultVar1 := s.ChannelStore.CreateDirectChannel(userId, otherUserId)
@@ -1569,7 +1569,7 @@ func (s *TimerLayerChannelStore) Save(channel *model.Channel, maxChannelsPerTeam
 	return resultVar0, resultVar1
 }
 
-func (s *TimerLayerChannelStore) SaveDirectChannel(channel *model.Channel, member1 *model.ChannelMember, member2 *model.ChannelMember) (*model.Channel, *model.AppError) {
+func (s *TimerLayerChannelStore) SaveDirectChannel(channel *model.Channel, member1 *model.ChannelMember, member2 *model.ChannelMember) (*model.Channel, error) {
 	start := timemodule.Now()
 
 	resultVar0, resultVar1 := s.ChannelStore.SaveDirectChannel(channel, member1, member2)