Simplify error message when field cannot be found on update

### Please avoid duplicates

- [X] I checked [all open bugs](https://github.com/gr2m/github-project/issues?q=is%3Aissue+is%3Aopen+label%3Abug) and none of them matched my problem.

### Reproducible test case

_No response_

### Please select the environment(s) that are relevant to your bug report

- [X] Browsers
- [X] Node
- [X] Deno

### Versions

1.3.1

### What happened?

When I try to update a custom field "Type" I get the follow error message

> [github-project] "type" could not be matched with any of the existing field names: "Title", "Assignees", "Status", "Labels", "Linked Pull Requests", "Reviewers", "Repository", "Milestone", "Target Date", "NotType"

I think we should remove all the read-only fields, which would make this message much more readable

> [github-project] "type" could not be matched with any of the existing field names: "Title", "Status", "Target Date", "NotType"

### Would you be interested in contributing a fix?

- [ ] yes

No file, test, or entry point is named. Locate the update path that matches custom field names and builds the error message, then inspect how read-only fields are represented; done when those fields are excluded and the message lists only editable fields such as "Title", "Status", "Target Date", and "NotType".

Written by the indexing model from the issue text.