Fix/improve errors

[lukekugelman]

Creating this PR to align error handling in RTF with Javascript best practices. I was running into a problem with another library where it was expecting any exceptions thrown to be errors (with the property cause).

So the intent is to replace anytime a string was thrown in the past with a FiberError as to better handle any exceptions that RTF does not handle itself.

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[CodyJasonBennett]

I'm confused about the premise of this PR. Would it be sufficient to throw an Error here? Why abstract it into FiberError?

On a related note, I'm de-escalating text errors into warnings with #2395.

[lukekugelman]

Using Error would suffice. FiberError was created to allow developers to differentiate and handle RTF specifically if desired (by checking instanceof).

This is still needed for the other errors that are thrown by RTF, apart from the text errors.