MM-24135: Migrate AppError from SaveChannel/channel_store.go (#14299)

* 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

* Made all appError origins to be CreateChannel

* Remove typed internal error

* Fix translations

* fix layer generation

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

store/errors.go

@@ -0,0 +1,73 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package store
+
+import (
+	"fmt"
+)
+
+// ErrInvalidInput indicates an error that has occured due to an invalid input.
+type ErrInvalidInput struct {
+	Entity string      // The entity which was sent as the input.
+	Field  string      // The field of the entity which was invalid.
+	Value  interface{} // The actual value of the field.
+}
+
+func NewErrInvalidInput(entity, field string, value interface{}) *ErrInvalidInput {
+	return &ErrInvalidInput{
+		Entity: entity,
+		Field:  field,
+		Value:  value,
+	}
+}
+
+func (e *ErrInvalidInput) Error() string {
+	return fmt.Sprintf("invalid input: entity: %s field: %s value: %s", e.Entity, e.Field, e.Value)
+}
+
+// ErrLimitExceeded indicates an error that has occured because some value exceeded a limit.
+type ErrLimitExceeded struct {
+	What  string // What was the object that exceeded.
+	Count int    // The value of the object.
+	meta  string // Any additional metadata.
+}
+
+func NewErrLimitExceeded(what string, count int, meta string) *ErrLimitExceeded {
+	return &ErrLimitExceeded{
+		What:  what,
+		Count: count,
+		meta:  meta,
+	}
+}
+
+func (e *ErrLimitExceeded) Error() string {
+	return fmt.Sprintf("limit exceeded: what: %s count: %d metadata: %s", e.What, e.Count, e.meta)
+}
+
+// ErrConflict indicates a conflict that occured.
+type ErrConflict struct {
+	Resource string // The resource which created the conflict.
+	err      error  // Internal error.
+	meta     string // Any additional metadata.
+}
+
+func NewErrConflict(resource string, err error, meta string) *ErrConflict {
+	return &ErrConflict{
+		Resource: resource,
+		err:      err,
+		meta:     meta,
+	}
+}
+
+func (e *ErrConflict) Error() string {
+	return e.Resource + "exists " + e.meta + " " + e.err.Error()
+}
+
+func (e *ErrConflict) Unwrap() error {
+	return e.err
+}
+
+// TODO:
+// type ErrNotFound struct {
+// }