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/api/org_invite.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/grafana/grafana/pkg/api/dtos"
@@ -81,6 +82,7 @@ func AddOrgInvite(c *models.ReqContext, inviteDto dtos.AddInviteForm) Response {
 			if err == models.ErrSmtpNotEnabled {
 				return Error(412, err.Error(), err)
 			}
+
 			return Error(500, "Failed to send email invite", err)
 		}
 
@@ -181,6 +183,10 @@ func (hs *HTTPServer) CompleteInvite(c *models.ReqContext, completeInvite dtos.C
 	}
 
 	if err := bus.Dispatch(&cmd); err != nil {
+		if errors.Is(err, models.ErrUserAlreadyExists) {
+			return Error(412, fmt.Sprintf("User with email '%s' or username '%s' already exists", completeInvite.Email, completeInvite.Username), err)
+		}
+
 		return Error(500, "failed to create user", err)
 	}