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>

api4/apitestlib.go

@@ -881,7 +881,7 @@ func CheckErrorMessage(t *testing.T, resp *model.Response, errorId string) {
 	t.Helper()
 
 	require.NotNilf(t, resp.Error, "should have errored with message: %s", errorId)
-	require.Equalf(t, resp.Error.Id, errorId, "incorrect error message, actual: %s, expected: %s", resp.Error.Id, errorId)
+	require.Equalf(t, errorId, resp.Error.Id, "incorrect error message, actual: %s, expected: %s", resp.Error.Id, errorId)
 }
 
 func CheckStartsWith(t *testing.T, value, prefix, message string) {

api4/user_test.go

@@ -53,13 +53,13 @@ func TestCreateUser(t *testing.T) {
 	ruser.Username = GenerateTestUsername()
 	ruser.Password = "passwd1"
 	_, resp = th.Client.CreateUser(ruser)
-	CheckErrorMessage(t, resp, "app.user.save.existing.app_error")
+	CheckErrorMessage(t, resp, "app.user.save.email_exists.app_error")
 	CheckBadRequestStatus(t, resp)
 
 	ruser.Email = th.GenerateTestEmail()
 	ruser.Username = user.Username
 	_, resp = th.Client.CreateUser(ruser)
-	CheckErrorMessage(t, resp, "app.user.save.existing.app_error")
+	CheckErrorMessage(t, resp, "app.user.save.username_exists.app_error")
 	CheckBadRequestStatus(t, resp)
 
 	ruser.Email = ""