fix: http code if user already member in invite (#783)

Service won't return 5xx if a user is already a member of
organization while creating the invite.

Signed-off-by: Kush Sharma <thekushsharma@gmail.com>

.mockery.yaml

@@ -59,6 +59,10 @@ packages:
     config:
       dir: "core/organization/mocks"
       all: true
+  github.com/raystack/frontier/core/invitation:
+    config:
+      dir: "core/invitation/mocks"
+      all: true
   github.com/raystack/frontier/core/permission:
     config:
       dir: "core/permission/mocks"

core/invitation/invite.go

@@ -9,8 +9,9 @@ import (
 )
 
 var (
-	ErrNotFound   = errors.New("invitation not found")
-	InviteExpired = errors.New("invitation expired")
+	ErrNotFound      = errors.New("invitation not found")
+	ErrInviteExpired = errors.New("invitation expired")
+	ErrAlreadyMember = errors.New("user already exists in organization")
 )
 
 const (