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/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 = ""