Users: Improve conflict error handling (#26958)

* API: Improve error handling (#26934)

* New ErrUserAlreadyExists error has been introduced

* Create user endpoint returns 412 Precondition Failed on ErrUserAlreadyExists errors

* Make ErrUserAlreadyExists error message clearer

Co-authored-by: Emil Tullstedt <emil.tullstedt@grafana.com>

* Use errors.Is instead of equality comparator on AdminCreateUser handler

* Improve sqlstore/user test definition

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Improve sqlstore/user tests for ErrUserAlreadyExists cases

* Remove no needed string fmt and err declaration on sqlstore/user tests

* Code improvements for sqlstore/user tests

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Use err.Error() instead of sentinel error value on AdminCreateUser

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Add ErrUserAlreadyExists handling for signup & org invite use cases

Co-authored-by: Emil Tullstedt <emil.tullstedt@grafana.com>
Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

pkg/services/sqlstore/user.go

@@ -68,6 +68,11 @@ func CreateUser(ctx context.Context, cmd *models.CreateUserCommand) error {
 			cmd.Email = cmd.Login
 		}
 
+		exists, _ := sess.Where("email=? OR login=?", cmd.Email, cmd.Login).Get(&models.User{})
+		if exists {
+			return models.ErrUserAlreadyExists
+		}
+
 		// create user
 		user := models.User{
 			Email:         cmd.Email,