Make identity backend configurable for GraphAPI

In order to gradually switch to a new LDAP backend move
the existing code talking to CS3 into its own backend.

graph/pkg/service/v0/errorcode/errorcode.go

@@ -12,6 +12,11 @@ import (
 // ErrorCode defines code as used in MS Graph - see https://docs.microsoft.com/en-us/graph/errors?context=graph%2Fapi%2F1.0&view=graph-rest-1.0
 type ErrorCode int
 
+type Error struct {
+	errorCode ErrorCode
+	msg       string
+}
+
 const (
 	// AccessDenied defines the error if the caller doesn't have permission to perform the action.
 	AccessDenied ErrorCode = iota
@@ -66,6 +71,13 @@ var errorCodes = [...]string{
 	"unauthenticated",
 }
 
+func New(e ErrorCode, msg string) Error {
+	return Error{
+		errorCode: e,
+		msg:       msg,
+	}
+}
+
 // Render writes an Graph ErrorObject to the response writer
 func (e ErrorCode) Render(w http.ResponseWriter, r *http.Request, status int, msg string) {
 	innererror := map[string]interface{}{
@@ -85,6 +97,18 @@ func (e ErrorCode) Render(w http.ResponseWriter, r *http.Request, status int, ms
 	render.JSON(w, r, resp)
 }
 
+func (e Error) Render(w http.ResponseWriter, r *http.Request) {
+	status := http.StatusInternalServerError
+	if e.errorCode == ItemNotFound {
+		status = http.StatusNotFound
+	}
+	e.errorCode.Render(w, r, status, e.msg)
+}
+
 func (e ErrorCode) String() string {
 	return errorCodes[e]
 }
+
+func (e Error) Error() string {
+	return errorCodes[e.errorCode]
+}