MM-30041: Return correct error message for user save (#16117)

* MM-30041: Return correct error message for user save

We were collapsing all types of user conflict into a single
error message. Fixed it by inspecting the field of the invalidError type
and returning the correct message.

https://mattermost.atlassian.net/browse/MM-30041

```release-note
NONE
```

* Fix test errors

* Fix wrong message in test when comparing error messages

Co-authored-by: Rodrigo Villablanca <villa061004@gmail.com>

app/user_test.go

@@ -6,6 +6,7 @@ package app
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"image"
 	"image/color"
 	"strings"
@@ -18,6 +19,7 @@ import (
 	"github.com/mattermost/mattermost-server/v5/einterfaces"
 	"github.com/mattermost/mattermost-server/v5/model"
 	oauthgitlab "github.com/mattermost/mattermost-server/v5/model/gitlab"
+	"github.com/mattermost/mattermost-server/v5/store"
 	"github.com/mattermost/mattermost-server/v5/utils/testutils"
 )
 
@@ -361,6 +363,46 @@ func TestUpdateOAuthUserAttrs(t *testing.T) {
 	})
 }
 
+func TestCreateUserConflict(t *testing.T) {
+	th := Setup(t)
+	defer th.TearDown()
+
+	user := &model.User{
+		Email:    "test@localhost",
+		Username: model.NewId(),
+	}
+	user, err := th.App.Srv().Store.User().Save(user)
+	require.NoError(t, err)
+	username := user.Username
+
+	var invErr *store.ErrInvalidInput
+	// Same id
+	_, err = th.App.Srv().Store.User().Save(user)
+	require.Error(t, err)
+	require.True(t, errors.As(err, &invErr))
+	assert.Equal(t, "id", invErr.Field)
+
+	// Same email
+	user = &model.User{
+		Email:    "test@localhost",
+		Username: model.NewId(),
+	}
+	_, err = th.App.Srv().Store.User().Save(user)
+	require.Error(t, err)
+	require.True(t, errors.As(err, &invErr))
+	assert.Equal(t, "email", invErr.Field)
+
+	// Same username
+	user = &model.User{
+		Email:    "test2@localhost",
+		Username: username,
+	}
+	_, err = th.App.Srv().Store.User().Save(user)
+	require.Error(t, err)
+	require.True(t, errors.As(err, &invErr))
+	assert.Equal(t, "username", invErr.Field)
+}
+
 func TestUpdateUserEmail(t *testing.T) {
 	th := Setup(t)
 	defer th.TearDown()