[fix] Remove 'Shadow' from error message

[baseballyama]

fix: #3917

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[Rich-Harris]

So... this is a bit clunky but strictly speaking I think we need something like this, since standalone endpoint request handlers can return either a POJO or any valid Request body. Thoughts?

Suggested change

	throw new Error('Body returned from endpoint request handler must be a plain object');
	throw new Error('Body returned from page's endpoint request handler must be a plain object');

[baseballyama]

Essentially, the definition of the words may be ambiguous.
I think we should always stick to using the following three words which are the same as the document.

• Page
• Endpoint
• Standalone Endpoint

These are already described in the documentation, but I'm sure we can add additional explanations to the definitions of the words if users request them. Or do you want to drop the use of "Endpoint" and always use the word "Page's Endpoint"?
It's more clear to me, but I'm not sure if it's cool.

Personally, I don't have a problem with either of them, as it is important that the same word are used, but in any case, I suggest that the terminology be unified, including the codebase.

[Rich-Harris]

Yeah, I guess that's a fair point, we do already separate out 'standalone endpoints' and document the extra flexibility they have. So it's probably fine as-is. Thanks for the gut check!