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_test.go

@@ -568,6 +568,40 @@ func TestUserDataAccess(t *testing.T) {
 				So(query.Result.IsAdmin, ShouldEqual, true)
 			})
 		})
+
+		Convey("Given one user", func() {
+			const email = "user@test.com"
+			const username = "user"
+			createUserCmd := &models.CreateUserCommand{
+				Email: email,
+				Name:  "user",
+				Login: username,
+			}
+			err := CreateUser(context.Background(), createUserCmd)
+			So(err, ShouldBeNil)
+
+			Convey("When trying to create a new user with the same email, an error is returned", func() {
+				createUserCmd := &models.CreateUserCommand{
+					Email:        email,
+					Name:         "user2",
+					Login:        "user2",
+					SkipOrgSetup: true,
+				}
+				err := CreateUser(context.Background(), createUserCmd)
+				So(err, ShouldEqual, models.ErrUserAlreadyExists)
+			})
+
+			Convey("When trying to create a new user with the same login, an error is returned", func() {
+				createUserCmd := &models.CreateUserCommand{
+					Email:        "user2@test.com",
+					Name:         "user2",
+					Login:        username,
+					SkipOrgSetup: true,
+				}
+				err := CreateUser(context.Background(), createUserCmd)
+				So(err, ShouldEqual, models.ErrUserAlreadyExists)
+			})
+		})
 	})
 }