[A Github issue](https://github.com/angular/angular/issues/43821) about an arcane-sounding Forms error is one of the repo's top-ten most visited pages. This converts the error to `RuntimeErrorCode` and adds a dedicated guide to explain how to solve it.
PR Close#47969
Replace `new Error()` in a forms Validators function with `RuntimeError`, for better tree-shakability. Also, improve the error messages, and add documentation.
PR Close#46537
This commit performs some refactoring of the AbstractControl-based classes to employ shared `RuntimeError` class and also updates the code to avoid duplication and improve minification.
PR Close#44398